U.S. patent application number 11/917132 was filed with the patent office on 2011-01-13 for automotive adsorption heat pump.
Invention is credited to Benjamin J. Jones, Michael A. Lambert.
Application Number | 20110005267 11/917132 |
Document ID | / |
Family ID | 37532882 |
Filed Date | 2011-01-13 |
United States Patent
Application |
20110005267 |
Kind Code |
A1 |
Lambert; Michael A. ; et
al. |
January 13, 2011 |
AUTOMOTIVE ADSORPTION HEAT PUMP
Abstract
An adsorber unit has an outer shell, a plurality of internal
tubes extending through the shell for carrying heat transfer fluid,
each tube having outwardly projecting fins along its entire length,
and a solid adsorbent material in the shell surrounding the tubes
such that the fins project into the adsorbent material, the fins
being of a material (e.g., metal) of higher thermal conductivity
than the adsorbent material. Metal wool loosely packed inside the
tubes, or internal radial fins swaged into the tubes, increase
internal surface area thereby enhancing convective heat transfer.
Metal wool loosely packed between the external fins, or fine wire
metal coils lightly squeezed between the external fins, further
increase external surface area of the heat exchanger in contact
with the adsorbent thereby enhancing contact heat transfer.
Performance is enhanced because the external fins and wool or wire
coils transport heat more efficiently to all regions of the
adsorbent, and permit less non-adsorbent heat exchanger material
(e.g., metal) to be used for a given amount of adsorbent. Two or
more such units are used in an adsorption heat pump. This design
utilizes existing components (e.g., shell-&-tube heat
exchanger, internally and externally finned tubing, and metal wool
or wire coils) in a novel manner heretofore untried. In one
exemplary embodiment, automobile air conditioning, exhaust heat is
used to power such an air conditioner. The significant additional
power used by the mechanical compressor of an automobile (12%-17%
during commuting for subcompact to midsize cars) can be nearly
eliminated by powering the air conditioner with otherwise wasted
exhaust heat. The adsorbent is heated and cooled by light oil
(called Heat Transfer Fluid, HTF) which in turn is heated and
cooled by exhaust and fresh air. Such indirect heating and cooling
achieves the required efficiency, and allows using phase change
material (e.g., wax) to store and therefore fully utilize exhaust
heat. A refrigerant reservoir is included which provides immediate
cooling after start-up of a cold engine, while the exhaust system
and heat pump are still heating up in order to start pumping
refrigerant. Eliminating the mechanical compressor increases fuel
mileage by 14-18% for midsize, compact, MS and subcompact cars, or
4.6-6.0% annually, given a four-month cooling season.
Inventors: |
Lambert; Michael A.; (San
Diego, CA) ; Jones; Benjamin J.; (Escondido,
CA) |
Correspondence
Address: |
The Law Office of Jane K. Babin;Professional Corporation
c/o Intellevate, P.O. Box 52050
Minneapolis
MN
55402
US
|
Family ID: |
37532882 |
Appl. No.: |
11/917132 |
Filed: |
June 12, 2006 |
PCT Filed: |
June 12, 2006 |
PCT NO: |
PCT/US06/22905 |
371 Date: |
September 29, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60689664 |
Jun 10, 2005 |
|
|
|
Current U.S.
Class: |
62/476 |
Current CPC
Class: |
B60H 1/3201 20130101;
B60H 1/32 20130101; F25B 17/083 20130101; F25B 27/02 20130101; Y02A
30/274 20180101; B60H 1/32014 20190501 |
Class at
Publication: |
62/476 |
International
Class: |
F25B 15/00 20060101
F25B015/00 |
Claims
1. An adsorber unit for an adsorption heat pump, comprising: a
thermally insulated outer shell with a first end and a second end;
a first and second thermally insulated plenum having the first
plenum attached to the first end of the outer shell and the second
plenum attached to the second end of the outer shell, wherein each
plenum has at least one opening; a plurality of thermally
conductive tubes aligned longitudinally inside the shell, wherein
each tube comprises a plurality of projections extending from the
outside of the tube and projecting outwardly therefrom, wherein
each tube has a first and second ends; and wherein: the first end
of each tube is attached to the first plenum, and the second end of
the tube is attached to the second plenum; at least one solid
absorbent inside the shell surrounding the tubes; and at least one
refrigerant capable of being adsorbed and desorbed by the
adsorbent, inside the outer shell; wherein the outer shell, the
first and second plenums, and the outside surfaces of the tubes
form a closed system for containment of the absorbent and the
refrigerant.
2. The adsorber unit of claim 1, wherein at least one of the
thermally conductive tubes further comprises first thermally
conductive material filled between the projections for further
increasing the heat transfer surface area.
3. The adsorber unit of claim 2, wherein the first thermally
conductive material is metal wool, metal wires or carbon
fibers.
4. The adsorber unit of claim 1, wherein the solid adsorbent is one
or more selected from the group consisting of zeolites, silicas,
aluminas, active carbons, and graphites.
5. The adsorber unit of claim 1, wherein the refrigerant is one or
more selected from the group consisting of water, ammonium, and
low-boiling point alcohols, and halogenated refrigerants.
6. The adsorber unit of claim 1, wherein the projections have
higher thermal conductivity than the adsorbent.
7. The adsorber unit of claim 1, wherein the projections have a
thermal conductivity at least twice as large as the adsorbent.
8. The adsorber unit of claim 1, wherein the projections have a
thermal conductivity at least ten-fold as large as the
adsorbent.
9. The adsorber unit of claim 1, wherein the projections are thin
thermally conductive material.
10. The adsorber unit of claim 1, wherein the projections are
helically wound around the tube.
11. The adsorber unit of claim 1, wherein the projections are
annular metallic strips.
12. The adsorber unit of claim 1, wherein the projections are
fins.
13. The adsorber unit of claim 1, wherein the projections have a
total surface area at least as large as the total outside surface
area of the tubes.
14. The adsorber unit of claim 1, wherein the tubes have a greater
transverse thermal conductance than their longitudinal thermal
conductance.
15. The adsorber unit of claim 1, wherein the thermally conductive
material has a surface area at least as large as the outside
surface area of the tubes.
16. The adsorber unit of claim 1, wherein the outer shell is
cylindrical.
17. The adsorber unit of claim 1, wherein the adsorbent is powders,
particulates or granules.
18. The adsorber unit of claim 1, wherein each tube further
comprises second thermally conductive material inside of the tube
and contacting with the inside surface of the tube with minimum
blocking the flow of a heat transfer medium passing through the
tube.
19. The adsorber unit of claim 18, wherein the second thermally
conductive material is metal wool, metal wires, or carbon
fibers.
20. An adsorption heat pump comprising one or more adsorber units
of claim 1.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/689,664, filed Jun. 10, 2005.
BACKGROUND OF THE INVENTION
1. Automotive Cooling Technology Options
1.1 Typical Driving Scenario
[0002] In order to estimate the fuel savings afforded by an
adsorption air conditioner, it is necessary to define the basis of
the estimation, which will be the typical commute to and from work.
In the USA, driving mileage is distributed 38% commuting, 35% for
running family businesses, and 27% for social, recreational, and
religious activities. The average yearly mileage has increased from
16,539 km in 1990 to 18,870 km in 1999 [1], so a round estimate of
19,312 km.yr.sup.-1 (12,000 miles.yr.sup.-1) is assumed for 2006.
Therefore, the average round trip commute is 30.6 km=(19,312
km.yr.sup.-1.times.38% commuting/240 work days).
[0003] Assuming the typical commute is 50% city driving and 50%
highway driving, and that the average city street speed is 56
km.hr.sup.-1 (35 miles.hr.sup.-1) and the average highway speed is
97 km.hr.sup.-1 (60 miles.hr.sup.-1), the typical commuter spends
16 minutes on city streets and 10 minutes on freeways each day.
Also, the typical commuter spends 62 hours per year idling (at stop
signs, traffic signals, freeway on ramps, and in traffic jams) in
rush hour traffic [2]. This equates to another 15 minutes per day.
Thus the total daily commute time is 16+10+15=41 minutes,
.apprxeq.20 minutes to work and .apprxeq.20 minutes returning
home.
1.2. Automotive Cooling Requirements and Current Mechanical
Compression Technology
[0004] The cabin of a car parked in the open for a couple of hours
on a sunny and warm, not necessarily hot, day will get very hot
because the large area windows admit sunlight but trap infrared
radiation emanating from the interior (i.e., the greenhouse
effect). This is called "hot soaking." Interior temperature can
easily reach 60.degree. C. on a warm (25.degree. C.) day, and
exceed 70.degree. C. on a truly hot (35.degree. C.) day [3].
[0005] FIG. 1 shows the effect of operating the air conditioner on
fuel mileage for three classes of automobiles (subcompact, compact,
and midsize) during highway cruising, city driving, and idling, and
includes the amount of waste heat generated during each scenario.
For example, the air conditioner of a midsize car must have about 7
kW capacity to cool the "hot soaked" cabin to a comfortable
temperature within 10 minutes after start-up [3, 4]. During the
initial 10 minute "surge cooling" interval, the air conditioner
runs continuously (i.e., 100% duty cycle), after which it runs
intermittently to maintain a comfortable cabin, remaining on about
1/3 of the time (33% duty cycle), providing an average of 2.3 kW
cooling. Thus, for the typical 20 minute commute, the average
cooling load is 4.7 kW [=(7 kW.times.10 min.+2.3 kW.times.10
min.)/20 min.].
[0006] The Coefficient of Performance for Cooling [COP.sub.C, a
measure of efficiency equal to cooling (kW) divided by work input
(kW)] of automotive air conditioners is quite low. Heat pumps
commonly exhibit COP.sub.C=3.8 or greater for state-of-the-art
stationary (e.g., residential) applications with a modest
temperature "lift" (T.sub.lift=T.sub.cond-T.sub.evap) of 45.degree.
C.-5.degree. C.=40.degree. C. However, COP.sub.C drops to about 2.1
for vehicles, because T.sub.lift is increased to about 58.degree.
C.-3.degree. C.=55.degree. C. to permit smaller condensers and
evaporators, and each .degree. C. increase in T.sub.lift results in
a 2% to 4% decrease in COP.sub.C [3.8(1-3%
avg..times..DELTA.T.sub.lift)=3.8(1-0.03.times.15.degree. C.)=2.1].
Mechanical compressors are compact and light with Specific Cooling
Power, SCP.apprxeq.1000 W.kg.sup.-1 [5].
1.3 Potential Fuel Savings
[0007] The fuel savings to be realized from an exhaust powered air
conditioner is that due to eliminating the parasitic power
consumption of the mechanical compressor. The overwhelming majority
(>97%) of light duty vehicles (cars, vans, pick-ups, SUVs)
employ spark ignition (Otto cycle), gasoline burning engines, with
state-of-the-art thermal efficiency of 30%. The remaining 70% of
the heat of combustion is dissipated as heat, about 35% via the
radiator and 35% in the exhaust at city and highway speeds where
ram air induction effectively cools the radiator [1]. At idle, a
larger portion of waste heat (.apprxeq.2/3=67%) is discarded via
the exhaust. Even the minimal 3.5 kW of exhaust heat from an idling
subcompact (see FIG. 1) should be enough to power a regenerative
(heat recycling) adsorption heat pump providing 1.7 kW cooling (at
33% duty cycle) needed to maintain cabin comfort after initial
surge cooling.
[0008] The average parasitic power drain by the compressor of a
midsize car during the typical commute is 2.3 kW, and is obtained
by dividing the average 4.7 kW cooling load by COP.sub.C=2.1, then
adding a conservative 2% for belt friction. This average of 2.3 kW
is 9.3% of the 25.0 kW needed to propel a midsize car in highway
cruising and 14.8% of the 15.6 kW required for city driving (FIG.
1). The idling engine of a midsize car requires about 3 kW to
overcome internal friction. Add another 0.7 kW for back EMF from
the alternator (at 40 A.times.14V/80% efficiency) with A/C off and
1.5 kW for resistance from the torque converter for a total of 5.2
kW. Thus, the compressor comprises an extra 44% load (=2.3 kW/5.2
kW) on an idling engine.
[0009] Therefore, the average midsize car with a mechanical
compressor consumes 16% more fuel when the air conditioner is used
during commuting.
16 min .times. ( 15.6 + 2.3 ) kW + 10 min .times. ( 25.0 + 2.3 ) kW
+ 14 min .times. ( 5.2 + 2.3 ) kW ( 16 min .times. 15.6 kW ) + ( 10
min .times. 25.0 kW ) + ( 14 min .times. 5.2 kW ) = 1.16 = 16 % ( 1
) ##EQU00001##
A heat pump powered by presently wasted exhaust would eliminate the
16% additional power needed to operate the mechanical compressor
during the typical commute. Viewed differently, an exhaust powered
heat pump will reduce fuel consumption an average of
[(116%-100%)/116%].apprxeq.14% during air conditioner operation.
Substituting values for compact and subcompact cars from FIG. 1
into the above equation yields fuel savings of 17% and 18%,
respectively.
[0010] Assuming the cooling season averages four months for the
USA, the annual fuel savings is .about.5% for small to midsize
vehicles. Europe as a whole is the second largest automobile
market, after the USA, but has significantly lesser need for
automotive cooling, although the percentage of new vehicles
equipped with air conditioning is rapidly increasing nonetheless.
For sunnier Southern European locations bordering on the
Mediterranean (Iberia, southern France, Italy, Greece, and the
Balkans), savings similar those for the Southern USA "Sunbelt"
would be expected, significantly greater than the nationwide
average of .about.5%. Greater than 5% savings would be expected in
the large market (rivaling Europe) comprised of equatorial South
America (principally Brazil, followed by Northern Argentina then
Venezuela and Columbia), Australia, South Africa, Saharan Africa
(principally Egypt), the Near East, Middle East, and Southeast Asia
(principally Singapore, Taiwan, and South Korea). But fuel economy
may not matter much in the Middle East. Although China and India
have the #1 and #2 populations (1/3 of the world's) and are rapidly
developing, consuming ever greater amounts of oil, reliable
inexpensive transportation is still the order of the day, so air
conditioning automobiles is not yet a significant consideration.
Central Africa has a huge population and large cooling requirement,
but it is a small automobile market with basic transportation as
the goal. Japan, although a major automobile market (fourth), has
minimal need for automotive cooling.
1.3 Utilizing Waste Heat
[0011] There are three potential uses for waste heat in a vehicle:
(a) cabin heating, (b) cabin cooling, and (c) electricity
generation, the latter of which could be used for heating and
cooling. Heating is already performed efficiently, compactly, and
economically by routing engine coolant through a small finned tube
heat exchanger (HEX) in the cabin air duct. The only drawback is
the long delay (5+ min.) during frigid weather between engine
start-up and effective cabin heating and defrosting.
[0012] Alternators are typically rated at 105 A.times.14 V=1.5 kW,
which equates to a mechanical load on the engine of about 2.0 kW,
assuming .eta.=75%. Average alternator load is about half rated
output, since most driving is done in daylight with the lights off,
the cabin fan is usually on low or medium instead of high, and the
wipers are seldom used. The fuel pump runs constantly, but the
thermostatically controlled radiator fan often shuts off at city
cruise speed. So the power drain by an alternator at 50% (50 A) of
rated output is about 2.0 kW.times.50%=1.0 kW.
[0013] A thermoelectric generator directly powered by exhaust heat
could conceivably replace the alternator, and power motors
connected to the water pump, power steering pump, and compressor.
The average power drawn by the compressor during a typical 20
minute commute is 1.6 to 2.3 kW (FIG. 1), equal or greater than for
all other ancillary equipment combined (.apprxeq.1.0 kW by the
alternator plus .apprxeq.0.7 kW by the water and power steering
pumps). So, eliminating the compressor provides the greatest boost
to efficiency.
[0014] Using a thermoelectric generator to power a motor driving a
compressor is only one method of eliminating its parasitic power
drain. There are a number of thermal effect devices which can
convert wasted exhaust heat directly into cabin cooling without
having to go through the intermediate step of producing electricity
with the attendant losses in efficiency. Alternative cooling
technologies are reviewed next to determine the best option.
1.5 Alternative Cooling Technologies
1.5.1 Stirling Cycle Cooling
[0015] Recent improvements in efficiency of reversed Stirling cycle
systems (achieving COP.sub.C=3.0) still do not approach the
efficiency of the best reversed Rankine cycle designs
(COP.sub.C.gtoreq.5). Also, a reversed Stirling cycle heat pump
requires work input. But it cannot be belt driven by the engine if
the goal is to eliminate the parasitic power loss associated with
cabin cooling. So the heat pump must be driven by a motor that is
powered by a thermoelectric generator. This combination of three
components would add considerable mass. Each energy transformation
from heat to electricity to mechanical work to heat (cooling) would
incur loss in efficiency.
[0016] A state-of-the-art design [6] is reported to have SCP=12
W.kg.sup.-1, with COP.sub.C=3.0, and T.sub.lift=20.degree. C.,
about one-third of the required T.sub.lift.apprxeq.55.degree. C.
Increasing T.sub.lift to the required range will markedly reduce
COP.sub.C and SCP. But, even ignoring this degradation of
performance, a reversed Stirling cycle heat pump capable of
delivering 7 kW of cooling would have a mass of 580 kg [=7000 W=12
W.kg.sup.-1], not accounting for the considerable mass of the
thermo-electric generator.
1.5.2 Absorption (Liquid-Vapor) Cooling
[0017] The major difference between the liquid-vapor absorption
chiller and the mechanical-vapor-compression heat pump is the
primary form of energy used to power the cycle. The vapor
compressor is replaced by a liquid pump, which requires a fraction
of the power (.apprxeq.4%) to pump the much denser liquid solution
of refrigerant and absorbent to high pressure. A burner or solar
collector or low quality heat from a power plant or industrial
process heats the "generator" causing refrigerant to desorb from
the absorbent. The most common refrigerant-absorbent pairs are
ammonia-water (NH.sub.3--H.sub.2O) and water-lithium bromide
(H.sub.2O--LiBr). Absorption systems attain COP.sub.C.ltoreq.0.65
to 0.70 for "single effect" heating cycles which do not recover and
reuse heat after it passes through the system. "Double effect"
heating or "heat recovery" (i.e., recycling of heat) yields up to
COP.sub.C=1.2, but such devices are bulkier, more complex, and
costlier.
[0018] Boatto et al. [4] constructed an automotive absorption
system. They had difficulties in designing major components to meet
geometrical and functional specifications for integration in a car.
High system mass was also a problem. Separation of the refrigerant
from the absorbent in the "generator" was strongly affected by
acceleration and vibration. They suggested that the best
refrigerant-absorbent pair was H.sub.2O--LiBr, but cautioned that
corrosion by the hot brine was a challenging problem. Boatto et al.
[4] concluded that preheating the brine with engine coolant
entailed too many complications, and so chose to remain with a
system employing single effect heating that used only exhaust heat
and yielded COP.sub.C.apprxeq.0.5.
1.5.3 Absorption (Solid-Vapor) Cooling
[0019] Solid-vapor adsorption is similar to liquid-vapor
absorption, except that the refrigerant is adsorbed onto a solid
desiccant (freeze dried) rather than absorbed into a liquid
(dissolved). The adsorption cycle is illustrated in FIG. 2 and
proceeds as follows: [0020] a. At state 1, a cool canister, or
adsorber, contains adsorbent saturated with a large fraction of
refrigerant at slightly below P.sub.evap. The cool adsorber is
heated and desorbs refrigerant vapor isosterically (i.e., at
constant total mass in the adsorber), thereby pressurizing it to
state 2, slightly above P.sub.cond. At this point vapor starts
being forced out the hot adsorber, through a one-way "check" valve
to the condenser. [0021] b. Isobaric heating desorbs more
refrigerant, forcing it out the adsorber and into the condenser
until state 3 is attained, whereat the adsorber is nearly devoid of
refrigerant. [0022] c. The hot adsorber is then cooled
isosterically (at constant total mass) causing adsorption and
depressurization, until the pressure drops below P.sub.evap (state
4) opening another check valve to allow vapor to enter the adsorber
from the evaporator. [0023] d. Isobaric cooling to state 1
saturates the adsorbent, completing the cycle.
[0024] Thus, the mechanical compressor can be replaced with one or
more adsorbers. Cyclically and asynchronously heating and cooling
two or more adsorbers results in continuous cooling. Solid-vapor
heat pumps require a low quality heat source at typically 150 to
250.degree. C. Catalyzed automobile exhaust is usually at least
400.degree. C., even at idle.
[0025] Prototype adsorption (solid-vapor) systems with innovations
for recycling heat (up to 75% to date) have achieved COP.sub.C=1.2
[7, 8]. Also, adsorption systems can be designed to be unaffected
by acceleration and vibration, do not use highly corrosive brine,
and can be smaller for a given capacity than absorption systems. An
SCP of 220 W.kg.sup.-1 of adsorbent has been demonstrated [9] and
SCP=590 W.kg.sup.-1 adsorbent has been predicted [10].
[0026] Three adsorbent-refrigerant pairs have received the most
attention to date: zeolite (a class of highly nano-scopically
porous, alkali-alumino-silicate minerals with cage-like crystalline
lattices)-water, activated carbon-ammonia, and silica gel
(SiO.sub.2)-methanol (CH.sub.3OH).
1.5.4 Thermoelectric Cooling (Peltier Devices)
[0027] Although Peltier coolers can exhibit up to .eta.=45% [11],
they tend to have very low SCP of 10-25 W.kg.sup.-1 [12], as
compared with SCP=1000 W.kg.sup.-1 for a mechanical compressor [5].
Also, an automotive Peltier device would need an exhaust powered,
thermoelectric generator with efficiency .eta..ltoreq.5% [13] and
extremely low SCP=0.25 W.kg.sup.-1 [14]. The cost is $4 to $5 per
watt [11].
[0028] Thus, although simple in concept, a thermoelectric cooling
system would exhibit a mere .eta.=45%.times.5%=2.2%, far too low to
be powered by engine exhaust. It would provide no more than 0.44 kW
cooling for a compact car, as compared with 6 kW needed for surge
cooling (FIG. 1). The Peltier device would have a mass of 240 kg
(=6 kW/25 W.kg.sup.-1), and the thermoelectric generator, assuming
a very optimistic SCP=10 W.kg.sup.-1, would have a mass of 1300 kg
[=6 kW/10 W.kg.sup.-1)/45%].
1.5.5 Selection of the Most Promising Alternative Cooling
Technology
[0029] Of the four alternative technologies reviewed above,
adsorption (solid-vapor) cooling is the most promising. The other
three alternate cooling technologies are either unfeasible or not
as promising as solid-vapor adsorption cooling for reasons
described above.
2. Literature Review: State of the Art in Adsorption Heat Pumps
2.1 Simple Cycle Adsorption Heat Pumps Not Utilizing Heat
Recycling
[0030] Solid-vapor adsorption heat pumps were used in domestic
refrigerators and railroad cars in the 1920's and 1930's [15]. The
COP.sub.C for built and tested simple cycle (i.e., "single effect"
heating with no recycling of heat) adsorption heat pumps is 0.3 to
0.4 [9]. This is primarily due to the fact that heat rejected from
the adsorbent during the cooling phase was simply discarded. A
second reason for low COP.sub.C and SCP is that much of the mass
(the pressure vessel and its internal heat exchanger) is
non-adsorbing, or so-called "dead," mass that is unavoidably heated
and cooled with the adsorbent but contributes nothing to the
compression effect.
2.2 Recycling Heat to Increase COP.sub.C
[0031] COP.sub.C can be increased by recycling heat that is
necessarily rejected from the adsorbent bed being cooled by
transferring it to the adsorbent bed being heated, thereby reducing
the required external heat input ("make up" heat). A heat transfer
fluid (HTF: oil or glycol-water solution) is used to exchange heat
between beds. The effectiveness of heat recycling depends upon how
the heat is transferred from the bed being cooled to the bed being
heated, which is bounded by two extremes: (a) uniform temperature
heat recovery or "double effect" heating, and (b) "thermal wave"
regeneration described below.
2.2.1 Uniform Temperature Heat Recovery or "Double Effect"
Heating
[0032] Uniform temperature heat recovery or double effect heating
(FIG. 3) can reduce required "make-up" heat by about 40% in a
two-bed device, boosting COP.sub.C from 0.3 to 0.4 for single
effect (no recycling of heat) adsorption devices to 0.5 to 0.65
[0.3/(1-0.4)=0.5; and 0.4/(1-0.4).apprxeq.0.65]. Once the beds
reach equal temperature, double effect heating is no longer
possible (FIG. 3). Thus, the theoretical limit of heat recovery for
a two-bed device is 50%, but the aforementioned 40% is the
practical limit [9].
2.2.2 "Thermal Wave" Regeneration
[0033] "Thermal wave" regeneration results from employing moving
temperature gradients or "thermal waves" that traverse the
adsorbent beds to heat and cool them (FIG. 4) and was first
suggested by Tchernev and Emerson [7]. Thermal wave regeneration is
more efficient than uniform temperature heat recovery for a given
number of beds, since heat is transferred across a smaller
temperature difference, creating less entropy. Tchernev et al. [7,
8] demonstrated 75% thermal wave regeneration, elevating COP.sub.C
to about 1.2 [0.3/(1-0.75)=1.2]. The theoretical maximum efficiency
for thermal wave regeneration is 100% for an infinitesimal .DELTA.T
between HTF and adsorbent; however, the practical limit has been
estimated at 85% [8].
2.3 Synopsis of State-of-the-Art in Adsorption Heat Pumps
[0034] The current state-of-the-art in adsorption heat pumps has
been reviewed. Research groups in the United States, Italy, France,
China, and Japan have concentrated their efforts [17-29] on
devising improvements to the all-critical adsorbers, with the
primary goal of improving efficiency (COP.sub.C), which requires
increasing the percentage of recycled heat. Several investigations,
e.g., [17, 19-24, 26, 28], agree in identifying the two most
important parameters that must be maximized in order to increase
COP.sub.C: (1) the ratio of adsorbent ("live") mass to
non-adsorbent (inert or "dead") thermal mass C.sub.ads/C.sub.inert,
and (2) the NTU of the heat exchanger. Since they have been working
to maximize COP.sub.C for stationary applications, little effort
has been directed toward increasing SCP, which is at least as
important as COP.sub.C for transportation applications.
[0035] According to Lambert and Jones [16], some designs suffer
from low thermal mass ratio C.sub.ads/C.sub.inert, the first of the
two critical governing parameters identified above. And most also
suffer from low NTU, the second critical governing parameter,
because they do not distribute heat effectively due to small
contact area A.sub.contact between the HEX and a given volume of
adsorbent .sub.ads, such as the concentric tube configurations in
FIGS. 5 and 6. Thermal resistance due to small A.sub.contact is
exacerbated by the typically poor junction conductance k.sub.junc
between the metallic HEX and the nonmetallic adsorbent. FIGS. 7 and
8 show two configurations with substantially greater A.sub.contact,
a shell-&-tube type (FIG. 7) and a spiral tape type (FIG. 8)
devised by Wang et al. [25]. But these latter two types provide
much greater A.sub.contact at the expense of markedly lower
C.sub.ads/C.sub.inert. The one exception is the flat pipe
serpentine HEX winding between consolidated adsorbent tiles
designed by Tchernev et al. [7, 8], as shown in FIG. 9. However,
this design posed insurmountable problems in manufacturability,
reliability, and expense, owing to its delicate configuration and
sub-atmospheric pressure that allowed for air leaks into the
system.
[0036] Another limitation of previous designs is that none embody a
satisfactory method for increasing the poor thermal conductivity of
adsorbents k.sub.ads while retaining sufficient permeability to
refrigerant vapor. Consolidating adsorbents into bricks increased
k.sub.ads and marginally increased junction conductance h.sub.junc
but decreased vapor permeability by 3 to 4 orders of magnitude [7,
8, 18-20, 22, 23]. Binders used in consolidation occlude pores.
Some designs use a coiled tubing HEX inside beds of packed spheres
[17], resulting in a very low effective k.sub.ads. None of the
studies consider settling of the adsorbent particles, which may
cause adsorbent to lose contact with the heat exchanger.
Performance parameters for several investigations are compiled in
FIG. 10.
SUMMARY OF THE INVENTION
[0037] It is an object of the present invention to provide a new
and enhanced adsorption heat pump which is more efficient and
reliable, as well as less expensive.
[0038] It is a further object of the present invention to provide a
new and enhanced adsorber unit for use in an adsorption heat pump
or air conditioning system.
[0039] In one aspect of the present invention, an adsorber unit for
an adsorption heat pump is provided, which comprises: [0040] a
thermally insulated outer shell with a first end and a second end;
[0041] a first, and second thermally insulated plenum having the
first plenum attached to the first end of the outer shell and the
second plenum attached to the second end of the outer shell,
wherein each plenum has at least one opening; [0042] a plurality of
thermally conductive tubes aligned longitudinally inside the shell,
wherein each tube comprises a plurality of projections extending
from the outside of the tube and projecting outwardly therefrom,
wherein each tube has a first and second ends; and wherein: the
first end of each tube is attached to the first plenum, and the
second end of the tube is attached to the second plenum; [0043] at
least one solid absorbent inside the shell surrounding the tubes;
and [0044] at least one refrigerant capable of being adsorbed and
desorbed by the adsorbent, inside the outer shell;
[0045] wherein the outer shell, the first and second plenums, and
the outside surfaces of the tubes form a closed system for
containment of the absorbent and the refrigerant.
[0046] In another aspect, at least one of the thermally conductive
tubes of the adsorber unit further comprises first thermally
conductive material filled between the projections for further
increasing the heat transfer surface area. Suitable thermally
conductive materials include, but are not limited to, metal wool,
metal wires, carbon fibers, or mixtures thereof. The first
thermally conductive material may have a total surface area at
least as large as the total outside surface area of the tubes. The
first thermally conductive material may also have a total surface
area about at least twice, at least about three-times, at least
about 5-times, or at least about 10 times as large as the outside
surface area of the tubes.
[0047] In some embodiments, the solid adsorbent in the adsorber
unit may be one or more selected from the group consisting of
zeolites, silicas, aluminas, active carbons, and graphites. Various
types zeolites, silicas, aluminas, active carbons, and graphites
may be used in the present invention. The adsorbent may be powders,
particulates or granules.
[0048] In certain embodiments, the refrigerant may be one or more
selected from the group consisting of water, ammonium, and
low-boiling point alcohols, and halogenated refrigerants.
[0049] In certain embodiments, the projections have higher thermal
conductivity than the adsorbent. In particular, the thermal
conductivity of the projections is at least about twice, at least
three-times, at least about five-times, at least about 10-times, at
least about 20-times, or at least 100-times greater than that of
the adsorbent. The projections may have various geometries, such as
annular, triangular, rectangular, square, etc. The projections may
also be constructed from various thermal conductive materials, for
examples, but not limited to, aluminum, copper, gold, silver, iron,
or alloys. The projections may be thin metal strips. The
projections may also be fins. The projections may have a total
surface area at least as large as the total outside surface area of
the tubes. The projections may also have a total surface area about
at least twice, at least about three-times, at least about 5-times,
or at least about 10 times as large as the outside surface area of
the tubes.
[0050] In some embodiments, the tubes may have a greater transverse
thermal conductance than their longitudinal thermal conductance. In
particular, the transverse thermal conductance is at least about
1.2-times, at least about 1.5-times, at least about twice, at least
about 5-times, at least about 10-times greater than the
longitudinal thermal conductance.
[0051] In still another aspect, each tube of the absorber unit
further comprises a second thermally conductive material inside of
the tube and contacting with the inside surface of the tube with
minimum blocking the flow of a heat transfer medium passing through
the tube. Suitable thermally conductive materials include, but are
not limited to, metal wool, metal wires, carbon fibers, or mixtures
thereof.
[0052] In an exemplary embodiment, an adsorber unit for a heat pump
comprises a cylindrical outer shell containing a plurality of tubes
passing longitudinally through the shell. Both ends of the tubes
are connected to plenums (or manifolds), one external and fixed to
one end of the shell, the other internal and free to telescope
within the shell due to thermal expansion of the tubes relative to
the shell. The tubes carry heat transfer fluid (HTF) for cyclically
heating and cooling the adsorber unit. Each tube has numerous
outwardly projecting fins extending along its length which project
into solid adsorbent material surrounding the tubes and filling the
shell. The fins are of a material with higher thermal conductivity
than the adsorbent material.
[0053] In another exemplary embodiment of the invention, the fins
comprise thin, helically wound, annular strips of metallic material
such as carbon steel which are brazed to the outer surface of the
heat transfer fluid (HTF) tubes, and the adsorbent material may be
powdered or particulate graphite or carbon. However, alternative
metallic (e.g., stainless steel, aluminum, or copper) fins may be
used in other embodiments and any suitable solid adsorbent material
(e.g., zeolite or silica gel) may also be used. The fins in the
exemplary embodiment have two orders of magnitude greater thermal
conductivity than the adsorbent material, which is typically in
particulate or powder form.
[0054] In this invention, the fins will effectively conduct heat to
all regions of the adsorbent, increasing the heat transfer rate and
allowing for a relatively wide spacing between the tubes inside the
adsorber shell, reducing the non-adsorbent, or "dead," mass. The
fins have an order of magnitude greater surface area than the HTF
tubes, which counters the low conductance through the microscopic
contacts and vapor filled gaps between the fins and the particulate
or powdered adsorbent.
[0055] The fins may be aligned perpendicular to the longitudinal
axis of the HTF tubes (i.e., transversely). In the exemplary
embodiment, the fins are a continuous annular helix, with adjacent
annuli closely spaced, approximately two to three millimeters
apart, along the entire length of the adsorber. This results in a
high ratio of transverse to longitudinal conductance, which
promotes "thermal wave" regeneration of heat, resulting in a higher
Coefficient of Performance for Cooling (COP.sub.C) than afforded by
uniform temperature heat recovery. To further increase the surface
area of the heat exchanger, metal wool may be loosely packed
between the fins, or small (e.g., 3 mm) diameter coils of fine wire
may be lightly squeezed between the fins. The metal wool or fine
wire metal coils may be of copper, aluminum, or steel. They are
fused to the fins by diffusion bonding (slow welding over time at
elevated temperature near the melting point) or plating (e.g.,
electro- or electro-less nickel plating).
[0056] In an exemplary embodiment of the invention, metal wool
material, for example 10% by volume copper wool, may be inserted
into the HTF tubes running through the adsorber shells. The heat
transfer fluid (HTF) may be oil or glycol/water mixture. Copper has
at least 1000 times the thermal conductivity of oil or water/glycol
(400 W.m.K.sup.-1 for copper versus 0.1 W.m.K.sup.-1 for oil and
0.4 W.m.K.sup.-1 for a 50/50 water/glycol mixture). Thus, the
copper transports heat from the core regions of the HTF near the
center of the tubes to the inside surfaces of the tubes, increasing
convective heat transfer. The HTF flow rate is very low, so the
increase in pressure drop incurred by the copper wool inserts is
minimal, thereby negligibly affecting the pumping power required to
circulate the HTF around the circuit. Alternatively, an extruded,
asterisk shaped, metallic insert may be swaged into the HTF tubes,
with each spoke of the asterisk forming an internal radial fin.
These radial fins increase internal surface area, thereby
increasing convective heat transfer.
[0057] According to still another aspect of the present invention,
an adsorption heat pump is provided, which consists of at least two
adsorbers, each comprised of an outer shell, at least one tube for
carrying HTF extending through each shell, each tube having
outwardly projecting fins extending along its length, and a solid
adsorbent material in the shell surrounding the tube such that the
fins project into the adsorbent material, the fins being of a
material of higher thermal conductivity than the adsorbent
material, an HTF heater having an inlet end connected to the HTF
tube in a first adsorber and an outlet end connected the HTF tube
in the second adsorber, an HTF cooler having an inlet end connected
to the HTF tube in the second adsorber and an outlet end connected
to the HTF tube in the first adsorber, and a refrigerant loop
having an evaporator and a condenser connected between the adsorber
shells such that refrigerant fluid flows through the adsorbent
material in the shells.
[0058] Such a heat pump may be used for any cooling or heating
application, such as residential, commercial, industrial,
agricultural heat pumps or chillers, refrigerated trucks or
trailers, buses, trains, and ships, and vehicle air conditioning
systems. A vehicle air conditioning system using the heat pump and
enhanced adsorbers of this invention could be powered by exhaust
heat.
[0059] In an exemplary embodiment of a vehicle adsorption air
conditioning system powered by exhaust heat, one or more thermal
reservoirs for storing exhaust heat may be secured to the HTF
manifolds of the HTF heater. The thermal reservoirs contain a phase
change material (PCM) such as wax, molten zinc, or molten lithium
to store exhaust heat when the engine is running above idle. This
stored heat will be used after the engine is shut off to desorb
practically all refrigerant from the adsorbers for storage in the
refrigerant reservoir. Refrigerant from the reservoir is then used
to provide cooling immediately after start up of a cold engine.
[0060] There are additional advantages in an exhaust powered
automotive or other vehicle air conditioning system incorporating
the adsorption heat pump of this invention. An automotive or
vehicle adsorber is subject to shocks and vibrations which will
eventually pulverize rather fragile consolidated adsorbent bricks
into powder, which will tend to settle. The fins on the HTF tubes
in each adsorber shell will retain adsorbent powder, and even if
some settling occurs over time, the fins will effectively
distribute heat to the adsorbent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0061] FIG. 1: Effect of operating the air conditioner on
performance of three classes of vehicles.
[0062] FIG. 2: Thermodynamic cycle for adsorption.
[0063] FIG. 3: Uniform temperature heat recovery or "double effect"
heating.
[0064] FIG. 4: Temperature variation through adsorbers, HTF heater,
and HTF cooler for "thermal wave" regeneration.
[0065] FIG. 5: Isometric cutaway of a segment of concentric tube
adsorber of Guillerminot et al. [20, 22, and 23]. Heat transfer
fluid (HTF) flows through the inner tube, and the annulus are
filled with consolidated adsorbent.
[0066] FIG. 6: Longitudinal cross-section of concentric tube
adsorber used by Pons et al. [18, 19]. The inner tube is filled
with consolidated adsorbent, and HTF flows through the narrow
annulus.
[0067] FIG. 7: Longitudinal cross-section of the shell-&-tube
adsorber used by Cacciola et al. [28]. Thin layers of zeolite
mineral adsorbent (white) are synthesized on the metal tubes (dark
crosshatching), and HTF flows through the tubes, while refrigerant
vapor occupies the spaces in the shell.
[0068] FIG. 8: Adsorber with internal spiral tape HEX proposed by
Wang et al. [25].
[0069] FIG. 9: Serpentine flat-pipe HEX interleaved with tiles of
consolidated zeolite adsorbent that was devised by Tchernev et al.
[7, 8].
[0070] FIG. 10: Performance of regenerative adsorption heat
pumps.
[0071] FIG. 11: Isometric view of design option one, showing
mechanical details to counteract thermally induced stresses.
[0072] FIG. 12: Exhaust and fresh air ducting for design option
one.
[0073] FIG. 13: Adsorber for design option two: (a) external, (b)
cross section.
[0074] FIG. 14: Exploded view of an adsorber showing 7 helically
finned HTF tubes, internal telescoping plenum to relieve
thermo-mechanical stresses, and fixed external plenum. This
application employs 19 HTF tubes, also in a regular hexagonal
array.
[0075] FIG. 15: Transverse and longitudinal cross-sections of
adsorber. The longitudinal view is truncated; length to diameter
ratio is 3.78. Proportions are accurate.
[0076] FIG. 16: Layout of exhaust powered automotive adsorption
heat pump.
[0077] FIG. 17: Schematic diagram of exhaust powered automotive
adsorption heat pump.
[0078] FIG. 18: Schematic diagram of exhaust powered automotive
adsorption heat pump. Major components are labeled. Each fluid
stream is color-coded as indicated in the legend. Sensors are
indicated by the various circles containing the acronyms FM (flow
meter), PT (pressure transducer), and TT (temperature transducer)
over numerals, most of which are used for the prototype and not on
a production model.
[0079] FIG. 19: Cutaway view of inter-loop heat exchanger.
Adsorption loop refrigerant (NH.sub.3 or CH.sub.3OH) evaporates
inside the coiled tubing, and R-134a condenses on the outside of
the tubes.
[0080] FIG. 20: Adsorptivity (kg per kg=wt %) of selected
substances in various adsorbents at 1 atm and 25.degree. C.
[31-33].
[0081] FIG. 21: Thermo-physical properties of selected refrigerants
at 300 K and 1 atm.
[0082] FIG. 22: Some common metals and alloys for adsorber
construction.
[0083] FIG. 23: Temperature versus time for the adsorbent in the 3
adsorbers. At any instant, one adsorber is heated while two are
cooled. The cycle period is 10 minutes.
[0084] FIG. 24: Adsorber geometry, mass, and thermal capacitance
(or "mass") at T.sub.ads=147.degree. C.
[0085] FIG. 25: Exhaust parameters for subcompact car with
1.5-liter engine.
[0086] FIG. 26: Temperatures of HTF, adsorbent, and intermediary
metallic HEX (tubes, fins, and wool) during heating and cooling
phases, showing reversal of transverse temperature gradient from
HTF to metal to adsorbent. Temperatures shown are for city cruise
with corresponding Q.sup..cndot..sub.cool=3.33 kW.
[0087] FIG. 27: Performance of adsorption heat pump in subcompact
car with 1.5 Liter engine.
[0088] FIG. 28: Isometric view of the heat transfer fluid (HTF)
heater, showing oval HTF tubes, manifolds, corrugated fins, and
thermal reservoirs containing phase change material (PCM).
[0089] FIG. 29: Geometry of HTF heater.
[0090] FIG. 30: Thermal & fluidic performance of HTF heater for
subcompact car.
[0091] FIG. 31. Comparison of adsorption heat pump to mechanical
compression air conditioner.
DETAILED DESCRIPTION OF THE INVENTION
[0092] As used in this disclosure, the singular forms "a", "an",
and "the" may refer to plural articles unless specifically stated
otherwise. Furthermore, the use of grammatical equivalents of
articles is not meant to imply differences among these terms unless
specifically indicated in the context. Unless defined otherwise,
all technical and scientific terms used herein generally have the
same meaning as commonly understood by one of ordinary skill in the
art to which this invention belongs.
[0093] To facilitate understanding of the invention set forth in
the disclosure that follows, a number of terms are defined
below.
Nomenclature
[0094] A surface, contact, or cross-sectional area (m.sup.2) [0095]
c.sub.p specific heat (J.kg.sup.-1.K.sup.-1) [0096] C thermal
capacitance, or thermal "mass," C=m.times.c.sub.p (J.K.sup.-1)
[0097] C.sup..cndot. dynamic thermal capacitance,
C.sup..cndot.=m.sup..cndot..times.c.sub.p (W.K.sup.-1) [0098]
COP.sub.C coefficient of performance for cooling, a dimensionless
measure of efficiency [0099] D.sub.h hydraulic diameter (m) [0100]
h.sub.ads heat of adsorption (kJ.kg.sup.-1) [0101]
.DELTA.h.sub.evap latent heat gain in evaporator (kJ.kg.sup.-1)
[0102] k.sub.fg heat of vaporization (kJ.kg.sup.-1) [0103]
h.sub.HTF convection heat transfer coefficient of heat transfer
fluid (W.m.sup.-2.K.sup.-1) [0104] h.sub.junc junction thermal
conductance of metal to vapor filled adsorbent
(W.m.sup.-2.K.sup.-1) [0105] HEX heat exchanger [0106] HTF heat
transfer fluid [0107] k thermal conductivity (W.m.sup.-1.K.sup.-1)
[0108] L length of tube or fin (m) [0109] m.sup..cndot. mass flow
rate (kg.s.sup.-1) [0110] mf mass fraction of refrigerant in
adsorbent [0111] N.sub.tube number of HTF tubes in the adsorber
[0112] Nu.sub.D Nusselt Number [0113] NTU Number of Transfer Units
for a heat exchanger, dimensionless, NTU=U.times.A/C.sup..cndot.
[0114] P.sub.cond condenser pressure (kPa or MPa) [0115] P.sub.evap
evaporator pressure (kPa or MPa) [0116] PCM phase change material
for storing exhaust heat (e.g., wax, zinc, or lithium) [0117] Pr
Prandtl number [0118] Q.sup..cndot. heat rate (W) [0119] R thermal
resistance, R=1=(U.times.A) (K.W.sup.-1) [0120] Re.sub.D Reynolds
number based on tube or channel hydraulic diameter [0121] SCP
specific cooling power (W.kg.sup.-1) [0122] S.sub.y yield strength
(MPa) [0123] S.sub.ys shear yield strength (MPa) [0124] .DELTA.T
temperature difference (K) [0125] T.sub.ads,max maximum adsorbent
temperature (.degree. C.) [0126] T.sub.ads,min minimum adsorbent
temperature (.degree. C.) [0127] T.sub.cond condenser temperature
(.degree. C.) [0128] T.sub.evap evaporator temperature (.degree.
C.) [0129] T.sub.lift temperature difference between evaporator and
condenser, T.sub.lift.ident.T.sub.cond-T.sub.emp (K) [0130] U
overall heat transfer coefficient per unit area
(W.m.sup.-2.K.sup.-1) [0131] U.times.A overall heat transfer
coefficient for given area (W.K.sup.-1) [0132] V velocity
(m.s.sup.-1) [0133] .sup..cndot. volumetric flow rate
(m.sup.3.s.sup.-1) [0134] .delta..sub.ads average adsorbent
thickness (m) [0135] .epsilon..sub.HEX effectiveness of heat
exchanger [0136] .eta. conversion efficiency (e.g., thermoelectric,
Peltier, or electro-mechanical) [0137] .eta..sub.Carnot Carnot
thermodynamic efficiency [0138] .eta..sub.fin efficiency of annular
helical fins on HTF tubes [0139] .eta..sub.pin efficiency of metal
wool fiber "pin" fins between annular helical fins [0140]
.eta..sub.0 overall surface efficiency of HTF tubes, annular
helical fins, and metal wool [0141] .mu. dynamic viscosity
(N.s.m.sup.-2) [0142] .nu. kinematic viscosity (m.sup.2.s.sup.-1)
[0143] .rho. density (kg.m.sup.-3) [0144] .sigma. normal stress
(MPa) [0145] .tau. shear stress (MPa) [0146] .chi..sub.reg fraction
of heat that is regenerated
Subscripts
[0146] [0147] ads adsorbent [0148] avail available [0149] conv
convection [0150] cond condenser or conduction [0151] evap
evaporator [0152] exh exhaust [0153] f liquid ("fluid") refrigerant
[0154] g gaseous (vaporous) refrigerant [0155] HTF heat transfer
fluid [0156] in inlet or inner [0157] intake intake air to engine
[0158] long longitudinal [0159] out outlet or outer [0160] r
refrigerant [0161] s solid, or surface of tube or flow passage
[0162] trans transverse
3. Conceptual Design of and Innovations for Adsorbers
3.1 Adsorption Cycle as Applied to an Automobile
[0163] With a single adsorber, cooling is intermittent, which is
undesirable because it wastes much of the continuous supply of
exhaust heat. So, at least two adsorbers are needed for an
automobile. Multiple adsorbers beyond two can enhance COP.sub.C by
permitting incrementally more effective "thermal wave" regeneration
[21], but add volume and mass, decreasing SCP. Thus a compromise
must be struck between SCP versus COP.sub.C to satisfy constraints
on both.
[0164] COP.sub.C must be high enough to ensure adequate cooling
even for the worst case scenario of a subcompact car idling for an
extended duration (i.e., traffic jam), since it has the largest
ratio of cooling load to exhaust heat. Maintaining an already surge
cooled cabin at a comfortable temperature requires 1.7 kW cooling
(FIG. 1). Assuming a realistic 80% of the 3.5 kW available exhaust
heat can be extracted (2.8 kW), the required COP.sub.C=1.7 kW/2.8
kW.apprxeq.0.60, which can be accomplished with uniform temperature
"double effect" heating.
[0165] Switching each adsorber from cooling to heating and vice
versa incurs some degree of time lag and reduction in COP.sub.C,
because the temperature gradient from HTF to internal metallic heat
exchanger to adsorbent must be reversed. This effect can be
quantified and further discussion is deferred to the detailed
thermal analysis in Section 8.1.4.
[0166] For a given configuration, SCP and COP.sub.C are inversely
proportional. But both SCP and COP.sub.C are directly proportional
to NTU and inversely proportional to the fraction of "dead" mass.
Thus, the fundamental objectives are maximizing NTU and minimizing
dead mass.
3.2 Conceptual Design Options
[0167] Two configurations have been devised. Option One employs
direct heating and cooling of the adsorbers by exhaust and fresh
air. Option Two utilizes indirect heating and cooling of the
adsorbers by a liquid HTF (oil) which has been heated by exhaust
and cooled by fresh air in a pair of finned tube, compact heat
exchangers similar to an automotive radiator.
[0168] The adsorbers are shell-&-tube type, which is readily
manufactured, cost effective, can withstand the relatively high
operating pressure if ammonia is the chosen refrigerant, and incurs
proportionally low dead mass.
3.2.1 Design Option One: Adsorbers Directly Heated by Exhaust and
Cooled by Air
[0169] FIG. 11 shows four shell-&-tube adsorbers divided into
two pairs. One pair is heated while the other pair is cooled. Hot
exhaust or cool ambient air passes alternately through the small
tubes to either heat or cool the adsorbent. The rear tube sheet is
split at its center plane to allow for differential thermal
expansion of the hot and cold adsorber pairs. The bellows of each
shell allow for differential thermal expansion with respect to the
exhaust/air tubes.
[0170] Option One requires large, complex ductwork for alternately
routing exhaust through either pair of adsorbers and cooling
ambient air through the other pair, as shown in FIG. 12. Each pair
of adsorbers requires a cooling fan and a damper to protect the fan
from exhaust.
3.2.2 Design Option Two: Adsorbers Heated & Cooled by Liquid
Heat Transfer Fluid (HTF)
[0171] Instead of utilizing four shorter adsorbers linked together
with plates and several external tie-rods as for Option One (FIG.
11), option two utilizes two longer adsorbers with circular end
caps and one central tie-rod to bear the load of pressurization
(FIG. 13). This configuration reduces dead mass. It also allows the
sizeable assembly of Option One (four linked adsorbers, inlet and
outlet ductwork, exhaust diverter vane, two fans, and two dampers)
to be separated into smaller devices that can be fit more easily
under the floor pan or in other locations.
[0172] Indirect heating and cooling via liquid HTF offers the
following advantages: [0173] Adsorbers can be cooled much closer to
ambient by liquid HTF than by air, allowing them to adsorb more
refrigerant. [0174] Liquid HTF pump requires only a small fraction
of the work of an air blower. [0175] Heat can be stored in a
thermal reservoir (e.g., wax).
[0176] Design Option Two was selected for its many advantages.
3.3 Innovations for Improving Performance Beyond the Current State
of the Art
[0177] The present invention relates to simultaneously increasing
both the adsorbent ("live") to inert ("dead") mass ratio
C.sub.ads/C.sub.inert as well as Number of Transfer Units (NTU).
These two parameters were identified by a number of previous
investigators as the critical factors governing performance (see
Section 2.3). This increases both COP.sub.C and SCP.
[0178] One aspect of the present invention is that of "activating"
the internal heat exchanger of the adsorbers. "Activating" means
increasing the surface area to volume ratio, and is usually used to
describe adsorbents, such as "activated" carbon, which is
pulverized to achieve enormous surface area to volume ratio. In
keeping with the notion of "activation," hundreds of thin metallic
annular fins are helically wound around then brazed to the HTF
tubes inside the adsorber shells (see FIGS. 12 and 13). The
benefits are several: [0179] (1) The metallic fins have thermal
conductivity, k.sub.fin, two to three orders of magnitude greater
than the conductivity of the powdered or granulated non-metallic
adsorbent, k.sub.ads. So the fins efficiently conduct heat to and
from all regions of the adsorbent. [0180] (2) The higher
conductivity of the fins with respect to the adsorbent allows for
relatively wide spacing between the tubes inside the adsorber
shell, thereby increasing the relative volume available for
adsorbent and the ratio C.sub.ads/C.sub.inert. [0181] (3) The
annular fins are closely spaced, 2-3 mm apart, thereby possessing
an order of magnitude greater surface area than the HTF tubes. This
augmented surface or contact area, A.sub.contact, counters the low
junction conductance, h.sub.junc, through the microscopic contacts
and vapor filled gaps between the fins and the powdered adsorbent.
[0182] (4) An automotive adsorber will be subjected to shocks and
vibrations which will eventually pulverize rather fragile
consolidated adsorbent bricks into powder. These hundreds of fins
will retain adsorbent powder, and even if some settling occurs over
time, the fins will still effectively distribute heat to the
adsorbent. [0183] (5) In order to further increase (or "activate")
the metal heat exchanger surface area in contact with the powdered
or granulated adsorbent, metal wool is loosely packed between the
metal annular fins (6-7% by volume, as compared with 4% in the
as-received condition), forming an interwoven network of wire "pin"
fins with myriad fin-wire and wire-wire contacts, which are then
fused together. Alternatively, small (.about.3 mm) diameter coils
of fine wire (100-150 .mu.m), resembling the springs in retractable
ball point pens, are helically wound between the annular fins. The
diameter of these fine wire coils is slightly larger than the
spacing between annular fins, so that the wire coils are lightly
squeezed between the fins, ensuring myriad fin-wire contacts. If
the tubes, external annular fins, and wire wool or wire coils are
made of copper or steel, they are bonded by nickel plating which
forms thermal bridges at the fin-wire and wire-wire contacts. If
the tubes, fins, and wire wool or wire coils are made of aluminum,
the myriad contacts can be diffusion bonded (i.e., slowly welded)
in a furnace maintained .about.100.degree. C. below the T.sub.melt
for a few hours. [0184] (6) In effect, a given volume of adsorbent
.sub.ads is spread very thinly over the enormous metal surface area
A.sub.contact, so that the distance from metal to any point in the
adsorbent, .sub.ads/A.sub.contact=d.sub.max, is a fraction of a
millimeter. This drastically reduces bulk thermal resistance
through the adsorbent. This design has an order of magnitude
smaller value of d.sub.max than any of the earlier designs (Table
1). [0185] (7) Since the contact area A.sub.contact is huge and the
conduction path through the adsorbent d.sub.max is tiny, the
adsorbent does not require consolidation to increase k.sub.ads or
h.sub.junc. This also avoids the problem of severely reduced (by a
factor of 10.sup.-3 to 10.sup.-4) vapor permeability due to
consolidation. As-received adsorbent powder or granules are simply
poured in and vibratory compacted to about 50% porosity, a readily
achievable target. [0186] (8) Spreading the metal internal
components very thin so as to increase their surface area also
reduces their required volume and mass. This increases the ratio of
adsorbent ("live") to inert ("dead") mass, C.sub.ads/C.sub.inert,
the second of the two critical performance parameters. [0187] (9)
Transverse (radial) resistance R.sub.trans through the tube walls,
fins, wire wool or wire coils, and thinly spread adsorbent is many
times smaller than longitudinal resistance R.sub.long along the
thin-walled HTF tubes. Thus heat flow is preferentially transverse,
giving rise to a significant longitudinal temperature gradient and
thereby permitting "thermal wave" regeneration. [0188] (10) Copper
wool can be loosely packed inside the HTF tubes (approx. 10% by
volume) to enhance convective heat transfer from the HTF to the
wall by a factor of 3-4. [0189] (11) A better performing, though
somewhat more expensive, alternative to copper wool is available.
Asterisk shaped extrusions with 6 to 12 "spokes", of aluminum,
copper, or carbon steel, are swaged into pre-tinned HTF tubes,
after which the assemblies are heated in an inert gas filled oven
to melt the brazing metal and fuse the fins to the tubes (see FIG.
14). These asterisk shaped inserts divide the single large round
flow passage into multiple wedge shaped lumens, reducing the
hydraulic diameter D.sub.h by a factor of 3-5. The "spokes"
increase internal surface area in contact with the HTF by a factor
of 2-4 and serve as radial fins to efficiently transfer heat
to/from the HTF. Thus, internal radial fins can increase convection
by an order of magnitude or more. Although multiple lumens are not
a new concept, usually only 4-6 are used or they are foregone
altogether to reduce pressure drop .DELTA.P and associated pumping
power. However, the HTF flow rate of this design is low, so
.DELTA.P is not a problem. [0190] (12) Phase change material (PCM)
such as wax, zinc, or lithium will be used to store exhaust heat
when the engine is running above idle. This stored heat will be
used after the engine is shut off to desorb all refrigerant from
the adsorbers for storage in the refrigerant reservoir in order to
provide cooling immediately after start up of a cold engine.
[0191] The rather expensive shell bellows (FIGS. 9 and 11) are
omitted in lieu of an internal telescoping HTF plenum (FIG. 14)
that allows for differential thermal expansion between the tubes
and shell.
[0192] These enhancements [(1) shell-&-tube HEX with annular
helical fins on the HTF tubes, (2) metal wool loosely packed
between the fins or fine wire metal coils wound between the fins,
(3) metal wool or radial fins inside the HTF tubes, and (4) phase
change material] for increasing both COP.sub.C and SCP are not
described in any of the literature. These enhancements are also
applicable to any adsorption cooling or heating application
including, but not limited to: NASA's proposed permanent manned
lunar habitat; residential, commercial, industrial, and
agricultural heat pumps and chillers; other transportation systems
such as refrigerated trucks and trailers, buses, trains, and
ships.
4. Embodiment Design of Automotive Adsorption Heat Pump
4.1 System Layout
[0193] The adsorption cooling system is depicted as integrated into
an automobile in FIG. 16. FIG. 17 is a basic schematic diagram of
the system, and FIG. 18 is a detailed schematic of the prototype
mounted on a cart. The system is comprised of three circuits, an
HTF loop, an ammonia (NH.sub.3) or methanol (CH.sub.3OH) adsorption
loop entirely exterior to the passenger cabin, and an R-134a
refrigerant loop transferring heat from the cabin to the exterior
NH.sub.3 (or CH.sub.3OH) loop. The R-134a loop can be eliminated by
pumping NH.sub.3 or CH.sub.3OH directly through the evaporator
inside the dash. Safety valves installed in the refrigerant tubing
would close automatically in the event of a leak to prevent
NH.sub.3 or CH.sub.3OH from entering the cabin.
4.2 Component Descriptions and Functions
[0194] Adsorbers: Contain powdered or granulated adsorbent and are
heated and cooled cyclically and asynchronously by hot and cold HTF
to pump refrigerant to the condenser and suck it from the
evaporator. Three adsorbers are shown in FIG. 16, rather than two.
This allows for more effective heating and cooling as is shown
quantitatively in Section 8.1.4 covering detailed design and
analysis.
[0195] Heat Transfer Fluid Heater: This is a counter-flow heat
exchanger wherein catalyzed exhaust heats HTF. It resembles an
enclosed radiator with multiple serpentine tube banks. As in a
typical radiator, the HTF tubes are oval shaped with thin
corrugated fins between them. This design exerts low back pressure
on the exhaust.
[0196] Thermal Reservoirs: Two thermal reservoirs store exhaust
heat in PCM (e.g., wax, zinc, or lithium) for drying all adsorbers
after the engine is shut off in order to fill the refrigerant
reservoir. The reservoirs are thin-walled, steel boxes measuring
approximately 40 cm.times.10 cm.times.3 cm, and are brazed to the
outboard surfaces of the HTF heater manifolds.
[0197] Heat Transfer Fluid Cooler: This is a radiator that
dissipates excess heat from the HTF to cool it near to ambient
before it is pumped into the adsorbers being cooled at the
moment.
[0198] Heat Transfer Fluid Pump, Tubing, and Expansion Tank: The
small, low power HTF pump comes after the HTF cooler in the
circuit, allowing for an inexpensive OEM fuel pump or engine oil
pump for a very small engine. The HTF tubes are insulated. The
expansion tank has an internal volume of about 0.6 liter. It is
almost empty when the HTF is cold and nearly full when the heat
pump is operating to make allowance for .about.12% expansion of the
HTF from ambient to mean operating temperature of
.about.160-170.degree. C.
[0199] Exhaust Bypass Pipe & Control Valves: Exhaust exiting
the catalytic converter ranges from 400.degree. C. at idle, to
450-500.degree. C. at city and highway cruise, to as high as
600.degree. C. for sustained operation at full throttle under heavy
load (e.g., uphill towing). Excess exhaust heat beyond that needed
to operate the heat pump could overheat the HTF. The bypass pipe
allows excess exhaust to be routed around the HTF heater. When the
heat pump is on, a servo motor controlled butterfly valve in the
bypass branch opens enough to tap off any excess exhaust. So,
although exhaust may reach 600.degree. C. in extreme cases, only a
small flow rate of such very hot exhaust would be allowed through
the HTF heater, not enough to overheat the HTF. When the heat pump
is off and the HTF is stagnant, the bypass valve is wide open, and
another solenoid controlled butterfly valve in the HTF heater
branch is closed, preventing overheating.
[0200] The bypass branch also ensures that excessive back pressure
will not result from trying to force all exhaust through the HTF
heater at or near full throttle.
[0201] Refrigerant Reservoir: This contains sufficient refrigerant
to provide immediate "surge cooling" during the initial 10 minute
interval after start up of a cold engine, while the HTF is being
heated in order to start thermally cycling the adsorbers and
pumping refrigerant.
[0202] Condenser: This is identical in size and shape to current
units, since likely adsorption refrigerants (e.g., NH.sub.3 or
CH.sub.3OH) have much better thermal properties than R-134a.
[0203] Evaporator: This is identical to current units, since it
also utilizes R-134a.
[0204] Inter-loop Heat Exchanger: This can be omitted if it is
decided to use a single refrigerant loop (e.g., NH.sub.3 or
CH.sub.3OH). It can be either a shell-&-tube HEX with internal
coiled NH.sub.3 (or CH.sub.3OH) tubing as shown in FIG. 19, or a
plate type HEX. Either type will be small, since it employs
two-phase heat transfer for both refrigerants (boiling NH.sub.3 or
CH.sub.3OH, condensing R-134a). A small, very low power pump
circulates R-134a through the nearly isobaric internal loop.
4.3 Cost Effective, Robust Design Elements
[0205] "Off-the-shelf" technology is employed for the condenser,
evaporator, inter-loop heat exchanger, refrigerant reservoir, and
HTF cooler, which are proven configurations with modest
modifications to account for particulars of this application. The
adsorbers have the readily manufactured, cost effective,
shell-&-tube configuration, which offers high strength and low
weight. The HTF heater resembles a finned tube radiator.
[0206] The adsorbers are cycled from ambient to .about.300.degree.
C., hot enough to deplete any adsorbent (zeolite, carbon, or silica
gel). The HTF heater is cycled from ambient to .about.375.degree.
C., far enough above a maximum HTF temperature .about.300.degree.
C. to promote effective heat recovery in a reasonably sized
package.
[0207] Depending upon the choice of refrigerant (e.g., NH.sub.3),
the adsorbers could be subjected to high pressure. For steel alloys
usually used in high temperature, high pressure applications,
allowable stress at .about.300.degree. C. is only modestly reduced
compared with allowable stress at room temperature, as per the ASME
Pressure Vessel & Piping Code [30]. Choices of specific metal
alloys for shell, tubes, fins, and wool are deferred to Section 5
after the refrigerant has been selected so as to include material
compatibility along with considerations of thermo-physical
properties, fatigue strength, creep resistance, manufacturability,
and cost.
[0208] Manufacturing methods are all cost effective and yield
durable products. [0209] a. No "exotic" or uncommon, usually
expensive, fabrication operations are involved. [0210] b. Nearly
all operations lend themselves to automation, and most tasks can be
performed by semi-skilled labor. For example, components can be
mass produced on common lathes and vertical mills (3-axis: x-y bed
with z-direction tool head) or CNC milling machines. [0211] c. All
tolerances are relatively loose. For example the length of heat
exchanger tubing inside the adsorbers need only be within .+-.2 mm
of nominal, and holes in the end plates for accepting these tubes
need be drilled only within .+-.0.25 mm. [0212] d. "As received"
finishes (e.g., machined, drawn, extruded) are suitable for all
components, foregoing secondary operations such as grinding,
lapping, and honing, which add cost. [0213] e. Pressure vessel
joints can be brazed in lieu of more expensive welding by skilled
labor, although automated (robotic) arc welding of seams on
adsorber shells may be more cost effective for high volume
production.
[0214] Materials and parts are all commonly available, being
produced in great numbers or bulk, and are inexpensive. [0215] a.
Adsorbents (zeolite, activated carbon, or silica gel) can be used
in as-received, powdered or granulated form and require no special
processing, such as consolidation into pellets or bricks, which
sacrifices vapor permeability for higher thermal conductivity,
trading one problem for another. Simple vibratory compaction to the
desired porosity will suffice. [0216] b. Heat exchanger tubing and
shells can be constructed from inexpensive carbon steel or modestly
more expensive low alloy steel or ferritic stainless steel.
Austenitic stainless steel, at somewhat greater expense, is an
option if higher corrosion resistance is required. [0217] c. Medium
to coarse grade metal wool and fine wire metal coils are
inexpensive. [0218] d. Simple, inexpensive, rugged ball check
valves are used to regulate flow of refrigerant. [0219] e. The HTF
pump is a low pressure (<400 kPa), medium capacity (5
liter.min.sup.-1) oil pump. [0220] f. The R-134a pump is of lower
pressure (<200 kPa) and capacity (1 liter.min.sup.-1) than the
HTF pump.
5. Material Selection
5.1 Adsorbent
[0221] Zeolites are alkali-alumino-silicate minerals containing
myriad nano-pores in their open, cage-like crystalline lattices
which permit them to adsorb large amounts of small, polar
molecules, especially water [31, 32]. Zeolites have low k on the
order of 0.1-1.0 W.m.sup.-1.K.sup.-1, which slows adsorption and
desorption, thereby limiting SCP [7, 8]. The most adsorbent
zeolite, type CaX, can adsorb 36%, 22%, and .apprxeq.30% (by
weight) water, ammonia, and methanol, respectively, as shown in
FIG. 20, which lists the adsorptivity, at atmospheric pressure and
room temperature, of various chemicals in several zeolite types and
a few other adsorbents.
[0222] The conductivity of activated (i.e., highly porous) silica
gel (SiO.sub.2) is similar to that of zeolites. Silica gel
completely desorbs most refrigerants at or below 150.degree. C.,
exhibiting a great affinity for methanol, adsorbing up to 50% by
mass, much greater than its affinity for water (33%) or ammonia
(13%) as shown in FIG. 20.
[0223] Graphite possesses very high k (1950 W.m.sup.-1.K.sup.-1
parallel to the lamellae, which resemble planar honeycomb
structures, and 5.70 W.m.sup.-1.K.sup.-1 perpendicular to the
lamellae). Another allotrope, carbon fiber, also has very high k of
up to 1100 W.m.sup.-1.K.sup.-1. Graphite and carbon can adsorb 62%
ammonia and 55% methanol [33], but very little water (see FIG. 20).
Graphite and carbon exhibit surface adsorption, as opposed to
zeolites which draw refrigerant molecules relatively deep (up to
100 .mu.m) within their crystalline lattices. Activated or expanded
graphite ("activated" and "expanded" meaning in the form of
microscopic powder) and carbon fibers have an enormous surface area
to volume ratio, enhancing their adsorptivity.
[0224] A coating of CaCl.sub.2 binds with ammonia at lower
temperatures and releases it higher temperatures, a complex
compound (chemi-sorption) reaction to augment surface Van der Waals
attraction and capillary condensation (physi-sorption) on the
activated carbon. Vasiliev et al. [33] demonstrate that CaCl.sub.2
coating on carbon fibers increases adsorptivity of NH.sub.3 by
.about.35%.
[0225] A compacted or consolidated mixture of zeolites and
activated graphite increases thermal conductivity and contact
conductance to the metallic HEX [7, 8, 18-20, 22, 23].
Consolidation involves mixing the adsorbent with a binder, usually
sodium meta-silicate (silica gel), followed by heating to drive off
the solvent. However, permeability, as compared with a bed of
spherical pellets, decreases by as much as a factor of 10.sup.-4
with increasing compaction and consolidation, severely impeding
vapor transport. This suggests an optimal intermediate density.
[0226] In order to maximize SCP, the best adsorbent should have the
greatest affinity for the chosen refrigerant. Activated graphite or
carbon is selected since it absorbs far more ammonia, the chosen
refrigerant as explained in Section 5.2 below, than any other
desiccant.
5.2 Refrigerant
[0227] The ideal refrigerant should be chosen for the prescribed
operating temperature range, T.sub.evap.apprxeq.3.degree. C. (if
water, benign) or -13.degree. C. (if ammonia or methanol,
hazardous), and T.sub.cond.apprxeq.65.degree. C. Corresponding
vapor pressures must not be too high, requiring overly robust
adsorber shell and tubing, nor sub-atmospheric, necessitating
inordinately large evaporator and condenser and making the system
prone to infiltration by air. As little as 1-2% non-condensable gas
(air) "poisons" two-phase heat transfer, halving the heat rate in
the condenser and evaporator. Relevant thermo-physical properties
of some candidate refrigerants are in FIG. 21, and characteristics
of more promising ones are described below: [0228] a. Water is
non-toxic, non-flammable, non-polluting, stable, and has the
highest latent heat among common substances (h.sub.fg=2257
kJ.kg.sup.-1@P.sub.atm). But, its vapor pressure is very low
[P.sub.cond=25 kPa at 65.degree. C., and P.sub.evap=0.8 kPa at
3.degree. C], requiring large condenser and evaporator. Moreover,
operating at sub-atmospheric pressure invites air "poisoning."
Operating the evaporator at just a few degrees above the freezing
point requires precise control, and the tubing must be drained to
prevent bursting when idle in frigid weather. [0229] b. Ammonia is
toxic, flammable in some concentrations (16-25%), non-polluting,
stable, and has the second highest latent heat (h.sub.fg=1368
kJ.kg.sup.-1@P.sub.atm) among common substances. When throttled
(isenthalpic) from a liquid at T.sub.cond,out=60.degree. C. to
T.sub.evap,in=-10.degree. C., .DELTA.h.sub.evap=958 kJ.kg.sup.-1.
Ammonia has P.sub.cond,in=2948 kPa at 65.degree. C., and
P.sub.evap,out=291 kPa at -13.degree. C. [0230] c. Methanol is
toxic, highly inflammable, non-polluting, unstable beyond 393 K,
and has the third highest latent heat (h.sub.fg=1101
J.kg.sup.-1@P.sub.atm) among common substances. When throttled from
60.degree. C. to -10.degree. C., .DELTA.h.sub.evap.apprxeq.1100
kJ.kg.sup.-1. P.sub.cond,in=101 kPa at 65.degree. C., but
P.sub.evap,out.apprxeq.3 kPa at -13.degree. C., so, "poisoning" by
air is a possibility. [0231] d. Ethanol is similar to methanol, but
is surpassed by methanol in all relevant thermophysical properties.
Ethanol also has sub-atmospheric operating pressures. [0232] e.
Propane is relatively non-toxic (irritating at high concentration,
but can asphyxiate if it displaces too much O.sub.2), highly
inflammable, and non-polluting. It has practical operating
pressures (>P.sub.atm), but its thermal properties are inferior
to H.sub.2O, NH.sub.3, and CH.sub.3OH. [0233] f. Carbon dioxide is
also being explored as a refrigerant by some European automakers.
It is non-toxic, non-flammable, and non-polluting. But it operates
in a trans-critical cycle with very high P.sub.cond.apprxeq.10,000
kPa, requiring a thick-walled adsorber shell and tubing.
[0234] Ammonia possesses the combination of high adsorptivity in
activated carbon or graphite, high latent heat, and practical
super-atmospheric operating pressures that allow for compact
condenser and evaporator, yet reasonably robust adsorbers, making
it the best choice.
5.3 Pressure Vessel Metals
[0235] The highest operating temperature is 300.degree. C. for an
adsorber that is indirectly heated and cooled by HTF, design Option
Two selected in Section 3.2. Mechanical and thermo-physical
properties of several classes of metal alloys are summarized in
FIG. 22. [0236] a. Aluminum and its alloy have the fourth highest k
(behind Ag, Cu, and Au). 2000 series aluminum alloys (e.g., the
widely used 2024-T4, -T6, or -T8) retain strength at elevated
temperature better than other series (3000, 5000, 6000, 7000, and
8000), but even 2000 series have very little strength at
300.degree. C. Ammonia is slightly corrosive to corrosive on
aluminum [34], and anodization cannot guarantee protection. [0237]
b. Copper has very high k (second only to Ag), but its alloys
(brass and bronze) have only about 1/4 the k. Copper and its alloys
have high thermal mass (C=.rho..times.c.sub.p) and lack the
strength and creep resistance at 300.degree. C. necessary to make
them viable for the pressure vessel. Copper is also rather
expensive. Moreover, ammonia is highly corrosive to copper and its
alloys [34], and nickel plating cannot assure protection. [0238] c.
Of the three classes of stainless steels, martensitic types (e.g.,
alloy 410) exhibit the highest S.sub.y at 300.degree. C. and
highest k but are least corrosion resistant. Ferritic types (e.g.,
alloy 446) have relatively high k for stainless alloys, high
S.sub.y when heat treated, and cost less than austenitic and
martensitic types. Ferritic grades are used in exhaust systems.
Austenitic alloys (e.g., alloys 304 and 316) have lowest k and low
S.sub.y but are the most corrosion resistant and are impervious to
ammonia [34]. [0239] d. Annealed, low carbon steels (e.g., SAE
1010) have the highest k of ferrous alloys and modest S.sub.y when
heat treated. Low alloy steels (e.g., "workhorse" alloys 4130 or
4340) have nearly equal k and much higher S.sub.y, retaining nearly
all their room temperature strength at 300.degree. C. Both plain
(carbon) and low alloys steels exhibit good fatigue strength and
can endure long-term, cyclical pressurization of the adsorbers.
Ammonia is very slightly corrosive to mild (low carbon) steel [34].
Galvanization is infeasible, since NH.sub.3 is highly corrosive to
zinc. So nickel plating may be needed for long-term protection.
[0240] Thus, if NH.sub.3 is the chosen refrigerant, only ferrous
alloys are feasible for the adsorber: shell, HTF tubes, annular
fins, and wool, anything that comes into contact with ammonia.
Stainless steel 304 (SS304, austenitic, 18% Cr, 8% Ni) is selected
for the shell and HTF tubes, i.e., the pressure envelope. Annealed
low carbon steel (SAE 1010) is used for the annular helical fins
and wool or wire coils, since it has the highest conductivity of
ferrous alloys, thrice that of SS304 (k=53.5 versus 16.8
W.m.sup.-1.K.sup.-1). SAE 1010 is also relatively soft and easily
formed.
[0241] The assembled tubes, fins, and wool or wire coils are
electroplated with nickel, or electro-less nickel (Ni.sub.3P),
forming thermal bridges at the myriad fin-wool and wool-wool
contacts, thereby creating an interwoven network of heat transfer
paths that effectively distribute heat to every portion of
adsorbent. The nickel plating also limits very weak corrosion of
the non-structural fins and wool or wire coils to the small
proportion of inevitable pinholes.
[0242] The tubes and shells are brazed to the end plates with
nickel-silver alloy (melting point .about.650.degree. C.) that is
compatible with NH.sub.3 and has shear yield strength
S.sub.ys>300 MPa.
5.4 Heat Transfer Fluid (HTF)
[0243] Some mineral oil based HTF, e.g., Dow-Therm A [35], can be
used up to 400.degree. C., far above the maximum foreseeable
operating temperature of 300.degree. C. And at 300.degree. C. they
have modest vapor pressures of only a few atmospheres
(P.sub.sat,HTF.apprxeq.2-3 P.sub.atm). The low flow rate of HTF
requires only another atmosphere or two of pressure differential to
overcome pumping losses (.DELTA.P.sub.pump,HTF.apprxeq.1-2
P.sub.atm). Thus, HTF gauge pressures are only 2-3 P.sub.atm and
heater tubes and manifolds are subjected to only a small fraction
of their allowable stress even at elevated temperature.
6. Functional Requirements
[0244] Functional specifications estimated in Section 1.2 (surge
cooling capacity of 5 kW) and Section 3.1 (COP.sub.C.apprxeq.0.60)
for a subcompact car, the type of vehicle with the most demanding
performance requirements, are applied to sizing adsorbers,
refrigerant reservoir, and HTF heater.
6.1 Operational Scenario and the Requirement for a Refrigerant
Reservoir
[0245] Two of the most important design criteria for the adsorber
are the amount of refrigerant required to run the desired cycle,
which is based on .DELTA.h.sub.evap for NH.sub.3, and the amount of
adsorbent required to adsorb and desorb the ammonia at the required
rate flow rate m.sup..cndot..sub.r.
[0246] As described in Section 1.2, cooling a car that has been
sitting in the sun for several hours (called "hot soaking") to a
comfortable temperature requires nominally Q.sup..cndot..sub.cool=5
to 7 kW of cooling for 10 minutes [3, 4]. The "hot soak" scenario
also assumes the engine is "cold," i.e., left off for several
hours. Mechanical compressors run continuously only during this
initial 10 minute cool down period on hot summer days. Once the
cabin is cool, about 1/3 of full capacity or
Q.sup..cndot..sub.cool=1.7 to 2.3 kW, is needed to maintain cabin
comfort, depending upon thermostat setting and ambient temperature
and humidity. Table 1 lists the additional power required by the
mechanical compressor and the exhaust heat that could power an
adsorption heat pump.
[0247] It is assumed herein that 5 minutes are required after a
cold start to heat the exhaust piping, catalytic converter, HTF
heater, and HTF sufficiently to begin heating the adsorbers.
Therefore, a refrigerant reservoir is required to meet the demand
for surge cooling (5 kW for 10 minutes for a subcompact car) while
the exhaust system and heat pump are warming up. The reservoir is
situated in the refrigerant circuit between the condenser and the
thermostatic expansion valve or capillary tube (FIGS. 15 and 16),
and contains pressurized liquid ammonia.
6.2 Required Cooling Capacity of Adsorbers
[0248] The adsorbers should not only maintain comfort in an already
surged cooled cabin, requiring Q.sup..cndot..sub.cool=1.67 kW for a
subcompact car (FIG. 1), but must also recharge the refrigerant
reservoir within a reasonable amount of time to ensure it is full
when the engine is turned off. It is assumed the reservoir is
emptied within 10 minutes after startup, beyond which surge cooling
is no longer needed. Subtracting the aforementioned 5 minute delay
for warm up after a cold start from a typical 20 minute commute
(Section 1.1) means the heat pump would operate 15 minutes while
the engine is running, during which the reservoir would be
recharged.
[0249] The refrigeration required to surge cool the cabin (5 kW)
and maintain comfort (1.67 kW) for a subcompact car over the 20
minute commute is (5 kW.times.10 min.)+(1.67 kW.times.10 min.)=66.7
kW-min. If the reservoir is to be replenished at the end of the
commute, then the adsorbers must provide 66.7 kW-min. of cooling
while they operate (15 minutes). Thus the adsorbers must have a
capacity of 66.7 kW-min./15 min.=4.45 kW.
[0250] The liquid HTF system with a PCM thermal storage reservoir
permits residual heat to be used to desorb refrigerant from the
adsorber that is saturated when the ignition is turned off. This
allows the heat pump to operate an additional 5 minutes or so at
full cooling capacity, 20 minutes total. Thus the adsorber output
must be 66.7 kW-min./20 min.=3.33 kW.
[0251] During the initial 10 minute surge cooling interval, 50
kW-min. of cooling is required. The heat pump operates during the
latter half of this interval, after the initial 5 minute delay to
heat the exhaust system. So, the adsorption heat pump produces 5
min..times.3.33 kW=16.7 kW-min. of cooling during the 10 minutes
after starting a cold engine. The refrigerant reservoir must store
enough refrigerant to make up the difference between demand (50
kW-min.) and supply (16.7 kW-min.) during the 10 minute surge
cooling interval, which is Q.sub.cool,reservoir=33.3 kW-min.=2000
kJ. The amount of ammonia in the full reservoir is:
m.sub.r,reservoir=Q.sub.cool,reservoir/.DELTA.h.sub.evap=(2000.times.10.-
sup.3)/(958.times.10.sup.3)=2.09 kg (2)
At 60.degree. C., .rho..sub.r=0.545 kg.liter.sup.-1, so the
reservoir must have a volume of 3.83 liters.
[0252] Compact and midsize cars would require 20% and 40%,
respectively, more ammonia than the subcompact (hybrid) car
examined above (see FIG. 1).
6.3 Required Amount of Adsorbent
[0253] Three adsorbers, instead of two, may be employed to take
advantage of the fact that minimum 400.degree. C. exhaust (at idle)
can rapidly heat one adsorber, permitting the other two to be
cooled twice as long at half the rate (FIG. 23). A cooling rate
that is 1/2 the heating rate incurs 1/2 the .DELTA.T.sub.HTF-ads so
that the adsorbent can be cooled closer to ambient and adsorb more
NH.sub.3.
[0254] Cycle duration is set at 10 minutes and is divided into
thirds. Each adsorber is heated for 1/3 of the cycle
(.DELTA.t.sub.heating=3.33 min.=200 s) and cooled for the remaining
2/3 of the cycle (.DELTA.t.sub.cooling=6.67 min=400 s). Their phase
angles are evenly spaced at 0.degree., 120.degree., and
240.degree., so at any given instant, one adsorber is being heated,
while two are being cooled. (FIG. 23).
[0255] The amount of NH.sub.3 that must be expelled from each
adsorber during its heating phase is:
m.sub.r={dot over
(Q)}.sub.cool.times..DELTA.t.sub.heating/.DELTA.h.sub.evap=3330.times.200-
/(958.times.10.sup.3)=0.696 kg (3)
[0256] Dry activated carbon fiber at 25.degree. C. can be saturated
with up to 62% NH.sub.3 (FIG. 20) after 120 minutes [33]. Adding a
CaCl.sub.2 coating boosts adsorptivity to 85% NH.sub.3 [33]
(Section 5.1). The dynamic adsorption capacity is 32% at 25.degree.
C. for a 10 minute cycle (adsorption/desorption) without
CaCl.sub.2, and 44% with CaCl.sub.2. Carbon and CaCl.sub.2 begin
desorbing NH.sub.3 above 50.degree. C.
[0257] The minimum practical adsorption temperature is
T.sub.ads,min=95.degree. C., which is high enough above the highest
foreseeable T.sub.amb=50.degree. C. to permit adequate heat
rejection during the cooling phase. At 95.degree. C., the dynamic
capacity of activated carbon fiber is 24% without CaCl.sub.2
(mf.sub.max=0.24) and 32% with CaCl.sub.2 (mf.sub.max=0.32).
Carbon/CaCl.sub.2 are depleted of NH.sub.3 at
T.sub.ads,max=200.degree. C. (mf.sub.min=0), well below
T.sub.exh,min=400.degree. C. Therefore, in each adsorber, the
amount of activated carbon m.sub.ads required to hold m.sub.r=0.696
kg of NH.sub.3 is:
m.sub.ads=m.sub.r/(mf.sub.max-mf.sub.min)=0.696/(0.32-0.00)=2.18 kg
(4)
6.4 Safety Considerations
[0258] In the event of a leak or a rupture due to a collision,
ammonia is only slightly flammable. A material safety data sheet
(MSDS) [34] lists its flammability as 1 on a 0-4 scale
(non-combustible=0 to highly inflammable=4), stating: "Slightly
flammable in the presence of open flames and sparks. Narrow lower
to upper flammability limits (16 to 25%) makes ignition difficult."
Its auto-ignition temperature is quite high, 651.degree. C.
[0259] But NH.sub.3 is a hazard; concentrations above 200 ppm cause
severe irritation to mucous membranes. However, ammonia is highly
volatile and its vapor is only 60% as dense as air, meaning any
escaping vapor would dissipate very rapidly in the event of a
collision. Moreover, the ammonia reservoir is a strong pressure
vessel that can be further armored by enclosing it in a simple
corrugated cylindrical steel canister capable of withstanding
severe impact. The reservoir would be mounted in the engine
compartment and separated from the cabin by the firewall. Also, the
total quantity of ammonia needed (3.83 liters) is 8-10% of the
typical capacity (40-50 liters) of a subcompact's gasoline
tank.
7. Sizing Components
7.1 Adsorbers
[0260] The subcompact (hybrid) car presents the greatest design
challenge, since it has the lowest ratio of exhaust heat
Q.sup..cndot..sub.exh to required cooling capacity
Q.sup..cndot..sub.cool at idle and cruising conditions (see FIG.
1). So this vehicle is used as the basis for sizing
computations.
[0261] One of three identical adsorbers is illustrated in FIGS. 12
and 13. Their geometry is listed in FIG. 24. Each shell in made of
thin-walled stainless steel 304 (SS304). It contains 19 SS304 tubes
which are internally threaded to double their convective surface
area. Fine copper wool is loosely packed into the tubes at 10% by
volume. Alternately, extruded radial fins (resembling an asterisk)
of annealed mild steel (SAE 1010) can be swaged and/or brazed
inside smooth (unthreaded) tubes. Each tube has 120 external
annular helical fins of annealed mild steel (SAE 1010). Steel wool
(also SAE 1010) is loosely packed between the fins to 6% by volume
(compared with 4% by volume as received) to create myriad fin-wool
and wool-wool contacts, which are fused by electro- or electro-less
nickel plating.
[0262] After subtracting the volume of the tubes, fins, and wool,
each adsorber has .sub.ads=1.997 liters of space to accommodate
m.sub.ads=2.18 kg of activated carbon as determined in Section 6.3.
At full compaction .rho..sub.ads=2.21 kg.liter.sup.-1. But
activated graphite is very porous, and must not be firmly compacted
to maintain permeability. The total porosity, within individual
particles and between particles in the aggregate, is:
porosity=(.sub.ads-m.sub.ads/.rho..sub.ads)/.sub.ads=(1.997-2.18/2.21)/1-
.997=0.506=51% (5)
Adsorbers for the compact car and midsize car (FIG. 1) scale
proportionately.
7.2 Refrigerant Reservoir and Inter-Loop Heat Exchange
[0263] The reservoir is a capsule, cylindrical with hemispherical
caps, 152.4 mm in diameter and 300 mm long, and contains the needed
3.83 liters of NH.sub.3 computed in Section 6.2.
[0264] The inter-loop HEX (FIG. 19) links the NH.sub.3 circuit to
the R-134a circuit (FIGS. 14-16). It has a thin-walled cylindrical
aluminum shell: 152.4 mm OD, 146.3 mm ID, L.sub.shell=305 mm. It
contains 7 coiled mild steel tubes (6.35 mm OD, 5.64 mm ID,
L.sub.tube=3.6 m), 30 turns each, in a regular hexagonal pattern.
NH.sub.3 evaporates inside the tubes, and R-134a condenses on the
tubes.
[0265] The volume of the NH.sub.3 reservoir and inter-loop HEX is
comparable to the volume occupied by the compressor, its bracket,
and the accumulator. But, neither of the adsorption components need
be mounted on the engine, allowing greater flexibility in
placement.
7.3 NH.sub.3 Condenser, HTF Cooler, and R-134a Evaporator
[0266] At T.sub.cond,in=65.degree. C., P.sub.cond,in for NH.sub.3
is 55% greater than for R-134a. And ammonia corrodes aluminum. So
thin-walled mild steel or ferritic stainless steel tubing is used.
Although NH.sub.3 has six times higher k and .DELTA.h.sub.evap than
R-134a, the condenser is the same size as current units, since air
side heat transfer is unaffected, requiring the same fin area. The
HTF cooler resembles the NH.sub.3 condenser (both compact HEX), is
about the same size, and is beside the NH.sub.3 condenser in front
of the radiator (FIG. 16). The R-134a evaporator inside the dash is
unchanged.
8. Detailed Design & Analysis
8.1 Thermal Performance of Adsorbers
8.1.1 Temperatures
[0267] Adsorbent temperature limits are selected as
T.sub.ads,min=95.degree. C. and T.sub.ads,max=200.degree. C. in
Section 6.3. To ensure adequate cooling of the adsorbers,
T.sub.ads,min is 35 K above T.sub.HTF,min.apprxeq.60.degree. C.,
which is in turn 10 K above the highest foreseen
T.sub.amb=50.degree. C. And T.sub.HTF,max.apprxeq.265.degree. C. is
65 K higher than T.sub.ads,max=200.degree. C. to rapidly heat the
adsorbent, yet still far below the minimum (at idle) catalyzed
exhaust temperature of T.sub.exh=400.degree. C., so that the HTF
can be heated in a reasonably sized HEX.
8.1.2 Available Heat
[0268] The subcompact (hybrid) car described in FIG. 1 provides the
lowest ratio of exhaust heat Q.sup..cndot..sub.exh to required
cooling capacity V.sup..cndot..sub.cool, so it is used as the basis
for computations. Also, an idling engine (e.g., during a traffic
jam) generates the least exhaust heat. FIG. 25 lists T.sub.exh,
.sup..cndot..sub.exh, and Q.sup..cndot..sub.exh for a subcompact
car with a 1.5 liter engine for idling, city cruise, and highway
cruise.
[0269] At idle, Q.sup..cndot..sub.exh=3.5 kW when it is cooled from
T.sub.exh,in=400.degree. C. to the dead state, herein taken to be
the standard T.sub.dead=25.degree. C. The practical lower limit to
which exhaust can be cooled is T.sub.exh,out=100.degree. C., since
condensation (causing accelerated corrosion) will occur inside the
exhaust system if it is cooled further. Thus, assuming constant
c.sub.p,exh, the available heat Q.sup..cndot..sub.exh,avail that
can be extracted is:
Q . exh , avail = ( T exh , in - T exh , out ) ( T exh , in - T
dead ) .times. Q . exh = ( 400 - 100 ) ( 400 - 25 ) .times. 3.5 kW
= 2.8 kW ( 6 ) ##EQU00002##
8.1.3 Required COP.sub.C
[0270] The minimum COP.sub.C to maintain an already cooled cabin
(at 33% duty cycle) for a subcompact car is
Q.sup..cndot..sub.cool/Q.sup..cndot..sub.exh,avail=1.67 kW/2.8
kW=0.60, which is near the middle of the range of
COP.sub.C=0.50-0.65 achievable for uniform temperature heat
recovery. Thus, for the most adverse operating condition
conceivable, a subcompact car with the lowest ratio of
Q.sup..cndot..sub.exh,avail to Q.sup..cndot..sub.cool, idling for
an extended length of time in a traffic jam, the adsorption heat
pump can, if properly designed, maintain a cool cabin. Conversely,
when a mechanical compressor is engaged on an idling engine, the
throttle opens considerably to provide the additional 44% to 65%
power required to turn the compressor (FIG. 1).
[0271] The most difficult design objective is minimizing the size
and mass of the adsorber (i.e., maximizing SCP), while
simultaneously enhancing COP.sub.C enough to provide sufficient
cooling. A secondary concern is ensuring engine performance is not
adversely affected by increased back pressure while recovering
enough exhaust heat to power the heat pump. The compact car and
midsize car (or SUV) have better (larger) ratios ratio
V.sup..cndot..sub.exh,avail to Q.sup..cndot..sub.cool, requiring
lower COP.sub.C, making their designs somewhat less exacting.
8.1.4 Heat Transfer in Adsorbers
1. Refrigerant Flow Rate
[0272] For the adsorber being heated the NH.sub.3 flow rate is:
{dot over (m)}.sub.r={dot over
(Q)}.sub.cool/.DELTA.h.sub.evap=3.3000/(958.times.10.sup.3)=0.00348
kg.s.sup.-1 (7)
For each of the two adsorbers being cooled the flow rate is halved:
m.sup..cndot..sub.r=0.00174 kg.s.sup.-1.
2. Thermal Masses
[0273] Adsorber geometry, mass, and thermal capacitance (or thermal
"mass," C) are provided in FIG. 24. The mass fraction of adsorbed
ammonia ranges from mf.sub.max=32% at 95.degree. C. to
mf.sub.min=0% at 200.degree. C., for an average of mf=16% at
T.sub.ads=147.degree. C. This corresponds to 0.349 kg of adsorbed
ammonia with a thermal mass of 850 J.K.sup.-1 at c.sub.p=2438
J.kg.sup.-1.K.sup.-1 (average of 2290 J.kg.sup.-1.K.sup.-1 at
95.degree. C. and 300 kPa; and 2585 J.kg.sup.-1.K.sup.-1 at
170.degree. C. and 2000 kPa). Thus, the total sensible thermal mass
of each adsorber is the sum of the thermal masses of the solids:
metal, adsorbent, and average amount of solidified refrigerant.
C.sub.solid=m.sub.metalc.sub.p,metal+m.sub.adsc.sub.p,ads+0.5(mf.sub.max-
+mf.sub.min)m.sub.adsc.sub.p,r=C.sub.metal+C.sub.ads+C.sub.r
(8)
C.sub.solid=3507+2247+850=6604 J.K.sup.-1 (9)
3. Sensible and Latent Heat Rates
[0274] The total sensible heat rate for each adsorber during the
heating phase is:
Q . sens , heating = C solid .times. .DELTA. T ads .DELTA. t
heating = 6604 .times. 105 200 = 3467 W ( 10 ) ##EQU00003##
The heat of adsorption is h.sub.ads=1151 kJ.kg.sup.-1. The latent
{adsorption) heat rate per adsorber during the heating phase
is:
Q . ads , heating = m ads .times. ( mf max - mf min ) .times. h ads
.DELTA. t heating = 2.18 .times. ( 0.32 - 0 ) .times. ( 1151
.times. 10 3 ) 200 = 4015 W ( 11 ) ##EQU00004##
[0275] Therefore the total heat rate into the heated adsorber
during the heating phase is:
{dot over (Q)}.sub.heating={dot over (Q)}.sub.sens,heating+{dot
over (Q)}.sub.ads,heating=3467+4015=7482 W (12)
4. HTF Flow Rate
[0276] The chosen HTF is Dow Chemical's Dowtherm A.RTM. synthetic
(i.e., thermally stabilized) organic base oil with 400.degree. C.
limit [35]. At a mean of T.sub.HTF.apprxeq.160.degree. C.:
c.sub.p=1968 J.kg.sup.-1.K.sup.-1, .rho.=943.3 kg.m.sup.-3,
.mu.=0.54.times.10.sup.-3 Pa.s, k=0.1163 W.m.sup.-1.K.sup.-1,
.nu.=0.57.times.10.sup.-6 m.sup.2.s.sup.-1, Pr=9.1. At a tube
surface temperature of about 250.degree. C.,
.mu..sub.s=0.28.times.10.sup.-3 Pa.s.
[0277] The mean HTF flow rate through the heated adsorber is:
m . _ HTF = Q . heating c p , HTF .times. .DELTA. T ads = 7482 1968
.times. ( 200 - 95 ) = 0.0362 kg . s - 1 ( 13 ) ##EQU00005##
5. Convection in HTF Tubes
[0278] Re D = 4 m . _ HTF N tube .pi. D .mu. = 472 ( 14 )
##EQU00006##
[0279] There are 19 HTF tubes in each adsorber. Flow is laminar
(Re.sub.D<2300), for which Nu.sub.D=4.36 for fully developed
laminar flow in a circular tube with uniform surface heat flux.
However, Nu.sub.D is boosted because the boundary layer is still
developing even at the tube exit:
N _ u D = 1.86 ( Re D Pr L / D ) 1 / 3 ( .mu. .mu. s ) 0.14 = 10.4
( 15 ) h HTF = N _ u D k HTF D = 128 W . m - 2 . K - 1 ( 16 )
##EQU00007##
[0280] This value of h.sub.HTF is far too low and will severely
limit heat transfer. The convection coefficient can be multiplied
by inserting loosely packed bundles of fine copper wool in the
tubes. The large surface area of the copper wool makes for
effective heat transfer from the HTF to the wool, and the high
conductivity of copper transmits heat to the walls of the tube.
Experiments have demonstrated that 10% by volume of 00 gauge (40
.mu.m fiber diameter) copper wool boosts the convection coefficient
by a factor of .about.4 to h.sub.HTF.apprxeq.500
W.m.sup.-2.K.sup.-1.
6. Efficiency of Annular Helical Fins and Wool "Pin" Fins
[0281] The annular, helically wound fins are annealed mild steel
(SAE 1010) with k.sub.fin=53.5 W.m.sup.-1.K.sup.-1 at 150.degree.
C. A conservative value of junction conductance h.sub.junc=200
W.m.sup.-2.K.sup.-1 (See FIG. 10) is used for the contact between
metal and activated carbon and gas gap conductance to ammonia
vapor. The annular helical fins have an efficiency of
.eta..sub.fin.apprxeq.78%.
[0282] The wool "pin" fins are also of annealed mild steel (SAE
1010) and meander through the gap between adjacent annular fins
making random wool-fin contacts. The length of each pin fin is
estimated to be about twice the spacing between annular fins (2.29
mm), or .about.5 mm, which should be conservative. The wool pin
fins have .eta..sub.pin.apprxeq.50%. The overall surface efficiency
of tubes, annular fins, and wool is .eta..sub.0.apprxeq.59%:
.eta. 0 = .eta. tube A tube + .eta. fin A fin + .eta. pin A pin A
tube + A fin + A pin = 0.586 .apprxeq. 0.59 ( 17 ) ##EQU00008##
7. Conductance Through the Adsorbent
[0283] Loosely compacted, activated carbon filled with NH.sub.3
vapor is assumed to have a conservative k.sub.ads.apprxeq.1.0
W.m.sup.-1.K.sup.-1 (see FIG. 10). Maximum distance from any point
in the adsorbent to a metal surface is:
.delta. ads = ads A tube + A fin + A pin = 1.997 .times. 10 - 3
0.21 + 1.23 + 3.80 = 0.00038 m = 0.38 mm ( 18 ) ##EQU00009##
8. Overall Transverse Conductance
[0284] The series resistance consisting of convection within the
HTF tubes, conduction through the tube walls, annular fins, and
wool, conductance across the metal-adsorbent interface, and
conduction through the vapor filled adsorbent is:
R trans = R HTF + R tube + R junc + R ads ( 19 ) R trans = 1 h HTF
A conv + ln ( D out / D in ) 2 .pi. kL + 1 .eta. 0 h junc A junc +
.delta. ads k ads A junc = 0.00795 K . W - 1 ( 20 )
##EQU00010##
[0285] The overall heat transfer coefficient is:
U.times.A=R.sub.trans.sup.-1=125.9 W.K.sup.-1 (21)
9. Transverse Conductance Versus Longitudinal Conductance
[0286] In order to ascertain whether the adsorbers are heated and
cooled uniformly or non-uniformly via a "thermal wave," the
longitudinal resistance is compared to the transverse resistance.
The principal longitudinal conduction paths are the shell and tube
walls, with cross sectional area A.sub.cross=0.002126 m.sup.2.
Selecting a small segment of the adsorber, say 3% of its length,
R.sub.long is:
R long = 0.03 .times. L k tube & shell A cross = 0.03 .times.
0.3048 16.8 .times. 0.002126 = 0.256 K . W - 1 ( 22 )
##EQU00011##
For the same segment (3% of L), R.sub.trans is 1/0.03=33.3 times
larger than computed above in Equation (20); that is,
R.sub.trans=0.265 K.W.sup.-1. Thus, R.sub.long and R.sub.trans are
about equal over any given 3% segment of an adsorber. So, heat flow
will be predominantly transverse. This will result in a
longitudinal gradient as HTF flows through the tubes making for
thermal wave regeneration, which can yield the greatest COP.sub.C
(see Section 2.2).
10 Cooling Phase
[0287] The two adsorbers being cooled are done so at half the rate
of the adsorber being heated. So, over 1/3 of a cycle each of the
cooled adsorbers changes temperature by
0.5.times..DELTA.T.sub.ads=52.5 K (see FIG. 23). A cooling rate
that is half the heat rate incurs half the .DELTA.T.sub.HTF-ads
(see FIG. 26):
Q . sens , cooling = - Q . sens , heating 2 = C solid .times. ( 0.5
.times. .DELTA. T ads ) 0.5 .times. .DELTA. t cooling = - 6604
.times. ( 0.5 .times. 105 ) 0.5 .times. 400 = - 1734 W ( 23 ) Q .
ads , cooling = - Q . ads , heating 2 = - m ads .times. 0.5 ( mf
max - mf min ) .times. h ads 0.5 .times. .DELTA. t cooling = - 0.5
m . r .times. h ads = - 2007 W ( 24 ) Q . cooling = Q . sens ,
cooling + Q . ads , cooling = - 1734 - 2007 = - 3741 W ( 25 )
##EQU00012##
11. Transient Effects
[0288] When an adsorber is transitioned from heating to cooling or
vice versa, the transverse temperature gradient from HTF to
adsorbent must be reversed, which requires additional sensible
cooling or heating of the intermediary metal HEX (tubes, fins, and
wool). See FIG. 26. For a non-regenerative heat pump, the excess
sensible heat in the metal of the hot adsorber is simply discarded,
clearly reducing COP.sub.C. But for a regenerative heat pump,
excess sensible heat removed from the metal of the hot adsorber may
be transferred to the cool adsorber. Although the excess sensible
heat of the metal is recovered in a regenerative heat pump, the
time lag involved in doing so diminishes Q.sup..cndot..sub.cool,
thereby reducing COP.sub.C. Or, to maintain a given
Q.sup..cndot..sub.cool, greater Q.sup..cndot..sub.make-up must be
added, also reducing COP.sub.C, and resulting in larger
.DELTA.T.sub.HTF-ads.
[0289] Since the metallic components are at temperatures between
T.sub.HTF and T.sub.ads, reversing the transverse gradient does not
require cooling the metal all the way from +.DELTA.T.sub.HTF-ads to
-.DELTA.T.sub.HTF-ads. For the current prototype, despite
convection enhancements to counter low k.sub.HTF, 73% of total
thermal resistance R.sub.total is between the HTF and the tube
wall, meaning only 27% of R.sub.total is between the tube wall and
the adsorbent. So,
.DELTA.T.sub.metal-ads.ltoreq.0.27.times..DELTA.T.sub.HTF-ads; it
is actually 0.20.times..DELTA.T.sub.HTF-ads accounting for the
temperature variation in the metal from tube inner wall to fins to
wool. Since .DELTA.T.sub.metal-ads must be reversed (+ to -, or -
to +), the change is 2.times.(0.20.times..DELTA.T.sub.HTF-ads).
Every 1/3 cycle (200 s), the adsorber being heated is switched to
cooling, and the colder of the two being cooled is switched to
heating. At nominal Q.sup..cndot..sub.cool=3.33 kW during city
cruise:
Q.sub.transient,heating=2.times.0.20.times.C.sub.metal.times..DELTA.T.su-
b.HTF-ads,heating=2.times.0.20.times.3507.times.59.4=83,300 J
(26)
{dot over (
Q.sub.transient,heating=Q.sub.transient,heating/.DELTA.t.sub.heating=83,3-
00/200=417 W (27)
Q.sub.transient,cooling=2.times.0.20.times.C.sub.metal.times..DELTA.T.su-
b.HTF-ads,cooling=2.times.0.20.times.3507.times.29.7=-41,700 J
(28)
{dot over (
Q.sub.transient,cooling=Q.sub.transient,cooling/(0.5.times..DELTA.t.sub.c-
ooling)=-41,700/(0.5.times.400)=-208 W (29)
12. Temperature Difference Between HTF and Adsorbent
[0290] The temperature differences between the HTF and adsorbent
for heating and cooling are:
.DELTA. T HTF - ads , heating = Q . heating + Q . _ transient ,
heating U .times. A = 7482 + 471 125.9 = 62.7 K ( 30 ) T HTF , max
= T ads , max + .DELTA. T HTF - ads , heating = 200 + 62.7 = 262.7
.degree. C . ( 31 ) .DELTA. T HTF - ads , cooling = Q . cooling + Q
. _ transient , cooling U .times. A = - 3741 - 208 125.9 = - 31.4 K
( 32 ) T HTF , min = T ads , min + .DELTA. T HTF - ads , cooling =
95 - 31.4 = 63.6 .degree. C . ( 33 ) ##EQU00013##
T.sub.HTF,max and T.sub.HTF,min differ by only a few K from their
estimated values (265.degree. C. and 65.degree. C., respectively),
obviating any need to adjust thermo-physical properties.
13. Predicted COP.sub.C
[0291] For constant c.sub.p,HTF the fraction of heat regenerated
.chi..sub.reg is (see FIG. 26):
.chi. reg = ( T ads , max + .DELTA. T HTF - ads , cooling ) - T HTF
, min ( T HTF , max - T HTF , min ) = ( 200 - 31.4 ) - 63.6 ( 262.7
- 63.6 ) = 105 199.1 = 0.527 ( 34 ) Q . make - up = Q . HTF ( 1 -
.chi. reg ) = m . HTF .times. c p , HTF .times. ( T HTF , max - T
HTF , min ) ( 1 - .chi. reg ) ( 35 ) Q . make - up = 0.0362 .times.
1968 .times. ( 262.7 - 63.6 ) ( 1 - 0.527 ) = 6704 W ( 36 ) COP C =
Q . cool / Q . make - up = 3333 / 6704 = 0.497 ( 37 )
##EQU00014##
[0292] Q.sup..cndot..sub.make-up=6.70 kW is 89% of available
exhaust heat in city cruise, Q.sup..cndot..sub.exh,avail=7.50 kW
(FIG. 22), and 94% of Q.sup..cndot..sub.HTF,heater=7.09 kW that can
be recovered by the HTF heater (see Section 8.2). Similar
computations are performed for Q.sup..cndot..sub.cool=2.00 kW at
idle and Q.sup..cndot..sub.cool=4.00 kW in highway cruise. The
results for all three driving modes during a typical 20 minute
commute are in FIG. 27. The 5 minute warm-up period for the exhaust
system and heat pump is assumed to be split into 2 minutes during
idling and 3 minutes during city driving; hence the heat pump
operates for 5 of 7 minutes of idling and 5 of 8 minutes of city
cruising. The heat pump is run at Q.sup..cndot..sub.cool=2.00 kW
for 10 minutes after the engine is shut off to maintain high
COP.sub.C while recharging the NH.sub.3 reservoir. The cumulative
cooling Q.sub.cool=66.7 kW-min. is equal to the amount determined
in Section 6.2. Q.sub.make-up=120 kW-min. is 90% of
Q.sub.HTF,heater=133 kW-min. that can be recovered by the HTF
heater, allowing for 10% heat loss.
[0293] The average SCP=408 W.kg.sup.-1 of activated carbon, which
is 185% of the 220 W.kg.sup.-1 demonstrated by Miles et al. (1993)
[9] and 69% of 590 W.kg.sup.-1 predicted by Jones (1993) [10].
8.1.5 Validation of Analytical Thermal Model Against
State-of-the-Art Prototype in Literature
[0294] Tchernev et al. [7, 8] built a prototype that exhibited a
COP.sub.C=1.2. The values of .chi..sub.reg and COP.sub.C for
Tchernev et al. [7, 8] are computed from the analytical model
developed for the present design, modified for geometry, materials,
etc. Their bench top prototype produced 1759 W cooling.
T.sub.source=T.sub.HTF,max=478 K and T.sub.sink=T.sub.HTF,min=311
K, which are the extremes to which the HTF was heated and cooled.
It is estimated the consolidated zeolite had 35% porosity,
k.sub.ads.apprxeq.0.25 W.m.sup.-1.K.sup.-1 (silica fired brick),
and h.sub.junc.apprxeq.1000 W.m.sup.-2.K.sup.-1, relatively high
because springs were used to compress the stack of alternating
layers of zeolite tiles and copper serpentine HEX (see FIG. 9). The
analytical model yields .DELTA.T.sub.HTF-ads=22.4 K and
Q.sup..cndot..sub.make-up=1445 W. Predicted .chi..sub.reg=0.731 is
2.5% less than experimental .chi..sub.reg=0.75, and predicted
COP.sub.C=1.22 is only 1.7% above experimental COP.sub.C=1.2.
8.2 Heat Transfer and Exhaust Pressure Drop in HTF Heater
8.2.1 Configuration and Thermal Analysis of HTF Heater
[0295] The HTF heater is a cross-flow compact heat exchanger (FIG.
28), similar to an automotive radiator. Its geometry is listed in
FIG. 29. There are 128 oval tubes divided into 8 bundles of 16
tubes each. HTF flows through the 8 bundles in series, thereby
making 8 serpentine passes across the exhaust flow. The oval tubes
are stacked in 8 layers, interleaved with 9 layers of corrugated
fins. The HTF heater is fabricated from ferritic stainless
steel.
[0296] The analysis of a compact heat exchanger such as the HTF
heater is straightforward and covered in such handbooks as [36].
Results are listed in FIG. 30.
[0297] Effectiveness .epsilon..sub.HEX is listed in FIG. 30 for a
cross-flow heat exchanger with C.sup..cndot..sub.max (HTF) unmixed
and C.sup..cndot..sub.min (exhaust) mixed. The HTF heater can
recover Q.sup..cndot..sub.HTF,heater that is 94% to 97% of
Q.sup..cndot..sub.exh,avail. These values are also listed in FIG.
27 and compared with Q.sup..cndot..sub.make-up, showing that during
a 20 minute commute all exhaust needs to be routed through the HTF
heater.
8.2.2 Exhaust Pressure Drop in HTF Heater
[0298] The maximum allowable pressure drop in the exhaust system is
typically 4-8 kPa for normally aspirated engines, and 2-4 kPa for
turbocharged engines, The method for computing exhaust pressure
drop .DELTA.P.sub.exh through the HTF heater is also given in [36].
At the highest exhaust flow rate corresponding to highway cruise,
total .DELTA.P.sub.exh=0.40 kPa if the entire exhaust flow is
routed through the HTF heater (FIG. 30), which is 10-20% of the 2-4
kPa total back pressure allowed for turbocharged engines. Moreover,
the exhaust is quieted by appreciable expansion within the HTF
heater, so the muffler can be omitted. This should offset the small
.DELTA.P.sub.exh imposed by the HTF heater.
8.3 Mechanical Design of Adsorbers
[0299] The reservoir and adsorbers are pressure vessels designed in
accordance with the ASME Boiler and Pressure Vessel Code [13]. The
maximum operating pressure is P.sub.cond=2948 kPa with ammonia as
the refrigerant. The stress state is biaxial normal with
.sigma..sub.hoop=107.5 MPa, .sigma..sub.long=39.9 MPa, and .tau.=0.
The corresponding von Mises stress is:
.sigma.'= {square root over
(.sigma..sub.hoop.sup.2+.sigma..sub.axial.sup.2-.sigma..sub.hoop.sigma..s-
ub.axial+3.tau..sup.2)}=94.2 MPa (72)
[0300] The ASME Boiler and Pressure Vessel Code [13] prescribes
maximum operating stress as a function of temperature for each
approved alloy. In highway cruise, the HTF will reach
.apprxeq.300.degree. C. At 343.degree. C. (650.degree. F.), the
lowest temperature at which allowable stress data are listed for
SS304, .sigma..sub.arrow=97.2 MPa for seamless SS304 pipe, 3%
greater than .sigma.'=94.2 MPa. For proof testing to 150% of
P.sub.cond at room temperature, .sigma.'=141.3 MPa, which is 59% of
S.sub.y=241 MPa for annealed SS304. SS304 (18% Cr, 8% Ni) is also
quite corrosion resistant so wall thinning should not be a problem.
If so, then even more corrosion resistant SS316 (16% Cr, 12% Ni, 2%
Mo) can be substituted at modestly greater expense. The
nickel-silver braze in the joints has S.sub.ys.apprxeq.1300 MPa,
many times greater than the proof shear stress .tau.=9.2 MPa.
8.4 Control System
8.4.1 Computer and Sensors
[0301] The adsorption heat pump is controlled by the climate
control computer. Feedback loop control of the adsorbers utilizes
thermocouples installed on the adsorbers to monitor their
temperatures.
8.4.2 Actuators (Servo, Solenoid, and Check Valves)
[0302] The climate computer controls the solenoid valve manifold
(FIG. 16) that sequences the flow of HTF through the three
adsorbers.
[0303] A servo motor controlled butterfly valve in the bypass
branch of the exhaust pipe apportions exhaust through the HTF
heater and bypass pipe in order to provide enough heat to operate
the air conditioner. A second, solenoid controlled, butterfly valve
in the HTF pipe branch closes when the air conditioner is off to
prevent overheating the HTF, since the HTF pump is off when the air
conditioner is off. The bypass valve is normally open when its
servo is de-energized. If the servo fails, the valve remains open
to prevent overheating the HTF. The HTF heater valve is normally
closed when its solenoid is de-energized, i.e., heat pump is off.
If the solenoid fails, the valve closes to prevent overheating the
HTF.
[0304] At T.sub.HTF.apprxeq.160.degree. C. the HTF expands about
12% with respect to T.sub.amb. The total volume of HTF is about 5
liters, so the expansion tank must be about 0.6 liter. It is
connected via a tee to a cool segment of HTF tubing. The vapor
pressure of Dowtherm A.RTM. near T.sub.amb is negligible [9],
allowing the entire volume of the expansion tank to be
utilized.
[0305] A small solenoid valve installed in the high-pressure
refrigerant tubing between the reservoir and the thermostatic
expansion valve serves as the on/off valve to start or stop the
flow of refrigerant through the evaporator. Passive one-way "check"
valves prevent reversal of refrigerant flow from the condenser and
reservoir to the adsorbers or from the adsorbers to the evaporator.
The power drain of all servos and solenoids is insignificant
8.4.3 HTF Pump, R-134a Pump, and HTF Cooler Fan
[0306] For the HTF loop, .DELTA.P.sub.HTF=98 kPa at
.cndot..sub.HTF,max=4.0 liters.min.sup.-1, requiring 6.5 W of
pumping power. For the R-134a loop, .DELTA.P.sub.R134a=78 kPa at
.cndot..sub.R134a=0.78 liter.min.sup.-1, requiring 1.0 W of pumping
power. Assuming only 30% efficiency for small DC motors, the
electrical input is (6.5 W+1.0 W)/0.30=25 W.
[0307] The radiator fan also serves the HTF cooler, since it is in
front of the radiator. The extra pressure drop of air passing
through the HTF cooler is roughly equal to the extra pressure drop
through the NH.sub.3 condenser, about 100 Pa. For a required
airflow rate through the HTF cooler of about 1.0 m.sup.3.s.sup.-1,
the required extra fan power is 100 W, but only at idle and low
speeds. At city and highway cruise speeds, ram air induction allows
the radiator fan to turn off, so the average additional fan power
imposed by the HTF cooler during typical commuting is about 25 W,
which correlates to about an additional 40 W electrical load for a
fan motor with 63% efficiency.
[0308] Thus, the electrical power demand of the adsorption heat
pump is (25 W+40 W)/1620 W=4.0% of the average power drawn by the
mechanical compressor of a subcompact car during commuting (at
average 67% duty cycle).
9. Comparison of Adsorption & Mechanical Air Conditioners
[0309] The mass breakdowns of mechanical compression and adsorption
heat pumps are listed in FIG. 30 for all three types of vehicles
identified in FIG. 1, Part 1 (subcompact, compact, and midsize).
FIG. 31 also lists average vehicle mass, plus one occupant
(.about.78 kg), for all three classes considered. The adsorption
heat pumps are 40 to 52 kg heavier than their mechanical
counterparts. This equates to 3.5-3.6% of total vehicle mass. The
percentage reduction in fuel mileage due to extra mass is about 60%
of the percentage extra mass, since wind resistance accounts for
about 40% of the overall power requirement averaged over city and
highway cruise speeds. So the reduction in fuel mileage would be
2.0-2.2%. For commuting, the increase in fuel mileage is 14%, 17%,
and 18% for midsize, compact, and subcompact cars, respectively
(see Section 1.3). Assuming the air conditioner is used 1/3 of the
time (4 months per year) the increase in fuel mileage would be
about 4.6% to 6.0%. Thus, the overall benefit (reduction in
auxiliary power demand) of the adsorption air conditioner outweighs
the mass penalty by a factor of 2.3 (=4.6%/2.0%) for midsize, 2.7
(=5.7%/2.1%) for a compact, and 2.7 (=6.0%/2.2%) for subcompact
cars.
10. Conclusions
[0310] Using exhaust heat to power an automotive air conditioner
would virtually eliminate the substantial power demand of currently
universal mechanical compressors, thereby increasing fuel mileage
and reducing pollution.
[0311] Of available thermally powered cooling technologies,
adsorption (solid-vapor) heat pumps are smaller and lighter than
absorption (liquid-vapor), reversed Stirling, and Peltier coolers,
the last two of which would require bulky thermoelectric
generators.
[0312] An adsorption heat pump is feasible for the following
reasons: [0313] (1) It can potentially reduce fuel consumption by
14% to 18% when in use (for a 50/50 mix of city and highway driving
for midsize, compact, and subcompact cars), or 4.6% to 6.0%
annually if the air conditioner is used 4 months of the year. This
enhancement is diluted by the increased mass of the adsorption heat
pump as compared with a mechanical compressor. The benefit-to-cost
ratio in terms of fuel savings from eliminating the mechanical
compressor as compared with the increased mass of the adsorption
heat pump is 2.3:1 for midsize cars and 2.7:1 for compact and
subcompact cars. [0314] (2) The enhanced performance described
herein employs cost effective and proven components, materials, and
essentially new manufacturing techniques with no exotic
technologies or materials being used. [0315] (3) Its performance
can match that of mechanical vapor compression devices. For
instance, a refrigerant reservoir can provide immediate cooling
after start up of a cold engine, as is so for a mechanical
compressor. [0316] (4) A refrigerant reservoir is especially useful
for hybrid vehicles in which the engine is turned off during
idling. A mechanical compressor would require a 2.4 to 3.4 kW motor
that would add mass and drain battery charge, the latter of which
is at a premium. [0317] (5) There, is enough heat in the exhaust of
even a subcompact car to power an adsorption heat pump at
sufficient cooling capacity.
[0318] The examples set forth above are provided to give those of
ordinary skill in the art with a complete disclosure and
description of how to make and use the preferred embodiments of the
present invention, and are not intended to limit the scope of what
the inventors regard as their invention. Modifications of the
above-described modes for carrying out the invention that are
obvious to persons of skill in the art are intended to be within
the scope of the following claims. All publications, patents, and
patent applications cited in this specification are incorporated
herein by reference as if each such publication, patent or patent
application were specifically and individually indicated to be
incorporated herein by reference.
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